The possible existence of Earth-like Binary Planets

The possible existence of Earth-like binary planets is being described today at the American Astronomical Society's Division for Planetary Sciences meeting in Tucson, AZ. 

Two bodies, each of mass similar to Earth, can form a closely orbiting pair under certain conditions present during the formation of planetary systems.

This theoretical proposal is completely unlike the Earth-Moon system or Pluto-Charon, where the two bodies are very different in mass, and arises in some "kissing" collisions where two similar mass bodies encounter each other and become a bound system because of the energy lost in the strong tides raised on each other in the encounter.

The resulting binary can then persist for billions of years provided it forms well away from the central star, at half an astronomical unit (the distance between the Earth and Sun) or more.

This work was presented by undergraduate Keegan Ryan, graduate student Miki Nakajima, and Dr. David Stevenson of the California Institute of Technology in Pasadena, CA.

The result does not contradict existing data for planets around other stars but suggests that future data may uncover such systems.

This is the first such study to examine the possibility of terrestrial binary planets.

During the formation of terrestrial planets, large rocky bodies orbiting around a star occasionally get close enough to interact with one another.

If two bodies collide head on or obliquely, then this interaction typically results in accretion where the two bodies merge to form a larger one, perhaps leaving behind a disk of debris from which a moon can form.

This is the standard picture for how Earth got its moon and a possible explanation for Pluto's moon, Charon.

If the two bodies collide in a grazing manner but at high velocity, then the two bodies "hit and run" and separate from one another once again, failing to form a bound pair.

The research presented today searched for a middle ground, a scenario in which the interaction results in two large bodes that do not merge but still remain locked in orbit.

This configuration, termed a terrestrial binary planetary system, would necessarily evolve into a state where the two bodies are tidally locked and with the centers of the two planets being separated by only three or so planet radii.

NB: With orbital period being almost the same as day length for both planets.

There is a good reason to believe terrestrial binary planetary systems may be possible.

In a grazing collision the angular momentum is too high to be contained within a single rotating body (it would fission) and if the bodies barely touch then they could retain their identity.

However, it requires an encounter where the bodies are initially approaching each other at low enough velocity.

To test for this possibility, a simulation technique called Smoothed Particle Hydrodynamics (SPH) was utilized.

Smoothed Particle Hydrodynamics represents a body as a collection of tens of thousands of particles, and it has been used to study protoplanetary collisions as well as the giant impact hypothesis of the Moon's formation.

Using SPH, collisions between two rocky Earth-sized bodies were simulated, with impact velocity and impact parameter (a measure of how head-on a collision is) being varied and the output observed.

In the cases where the bodies underwent substantial collision, the scientists replicated previous results in which a binary system did not arise but a moon might form.

However, by including interactions where the bodies are close enough to undergo a large tidal distortion, initial conditions were found that led to a terrestrial binary planetary system.

TESS mission cleared for next development phase

This is a conceptual image of the Transiting Exoplanet Survey Satellite (TESS). 

Credit: MIT

NASA has officially confirmed the Transiting Exoplanet Survey Satellite (TESS) mission, clearing it to move forward into the development phase.

This marks a significant step for the TESS mission, which would search the entire sky for planets outside our solar system, known as exoplanets.

Designed as the first all-sky survey, TESS would spend two years of an overall three-year funded science mission searching both hemispheres of the sky for nearby exoplanets.

"This is an incredibly exciting time for the search of planets outside our solar system," said Mark Sistilli, the TESS program executive from NASA Headquarters, Washington.

"We got the green light to start building what is going to be a spacecraft that could change what we think we know about exoplanets."

"During its first two years in orbit, the TESS spacecraft will concentrate its gaze on several hundred thousand specially chosen stars, looking for small dips in their light caused by orbiting planets passing between their host star and us," said TESS Principal Investigator George Ricker of the Massachusetts Institute of Technology, Cambridge, Massachusetts.

During the third year, ground-based astronomical observatories would continue monitoring exoplanets identified earlier by the TESS spacecraft.

TESS is expected to find more than 5,000 exoplanet candidates, including 50 Earth-sized planets.

It will also find a wide array of exoplanet types, ranging from small, rocky planets to gas giants.

Some of these planets could be the right sizes, and orbit at the correct distances from their stars, to potentially support life.

"The most exciting part of the search for planets outside our solar system is the identification of 'earthlike' planets with rocky surfaces and liquid water as well as temperatures and atmospheric constituents that appear hospitable to life," said TESS Project Manager Jeff Volosin at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"Although these planets are small and harder to detect from so far away, this is exactly the type of world that the TESS mission will focus on identifying."

Now that NASA has confirmed TESS, the next step is the Critical Design Review in 2015. This would clear the mission to build the necessary flight hardware for launch.

"After spending the past year building the team and honing the design, it is incredibly exciting to be approved to move forward toward implementing NASA's newest exoplanet hunting mission,"Volosin said.

TESS is designed to complement several other critical missions in the search for life on other planets.

Once TESS finds nearby exoplanets to study and determines their sizes, ground-based observatories and other NASA missions, like the James Webb Space Telescope, would make follow-up observations on the most promising candidates to determine their density and other key properties.

By figuring out a planet's characteristics, like its atmospheric conditions, scientists could determine whether the targeted planet has a habitable environment.

"TESS should discover thousands of new exoplanets within two hundred light years of Earth," Ricker said. "Most of these will be orbiting bright stars, making them ideal targets for characterization observations with NASA's James Webb Space Telescope."

"The Webb telescope and other teams will focus on understanding the atmospheres and surfaces of these distant worlds, and someday, hopefully identify the first signs of life outside of our solar system," Volosin said.

New milestone in the search for water on distant planets - HAT-P-11b

An artist's concept of the silhouette of the extrasolar planet HAT-P-11b as it passes its parent star. 

The planet was observed as it crossed in front of its star in order to learn more about its atmosphere. 

Credit: NASA/JPL-Caltech

Astronomers have found water vapour in the atmosphere of a planet about four times bigger than Earth, in the constellation Cygnus about 124 light years, or nearly 729 trillion miles, from our home planet.

In the quest to learn about planets beyond our solar system, this discovery marks the smallest planet for which scientists have been able to identify some chemical components of its atmosphere.

The researchers' findings were published Sept. 25, 2014 in the journal Nature.

The team was led by University of Maryland Astronomy Professor Drake Deming, an expert in the study of exoplanets, or planets that orbit suns outside our own solar system.

The finding of water vapour and hydrogen in the atmosphere of the exoplanet HAT P-11b is not only an astonishing piece of long-distance detective work, based on analyses of observations by three different NASA telescopes.

It also suggests that astronomers' ideas about how the planets formed appear to hold true for other planetary systems, as they do in our own.

How do scientists detect water in distant exoplanets? They use a quirk of light that happens when a planet transits, or passes in front of, its host star.

Material in the planet's atmosphere absorbs some of the star's light, and that makes the planet appear bigger. similar to the way our sun seems bigger at sunset, when we are looking to the horizon across a broad swath of Earth's atmosphere.

By plotting changes in the exoplanet's size, and relating them to the wavelength of electromagnetic radiation that the telescope observes, astronomers get a graph that shows how much of the star's radiation the planet's atmosphere is absorbing.

The shape of that graph, called a transmission spectrum, can reveal what chemicals are present in the atmosphere.

The bigger the planet, the more obvious are the changes in the planet's size during its transit across its host star.

Astronomers have used this technique to describe the atmospheres of several giant planets, the size of our solar system's Jupiter.

In this study, the team wanted to analyze the atmosphere of a significantly smaller planet. The team chose HAT P-11b, which was discovered by the Hungarian-made Automated Telescope (HAT) network.

It's about four times the radius of Earth and about 26 times Earth's mass. Compared to planets in our solar system,

HAT P-11b is closest in size to Neptune, but it is much closer to its host star and therefore much hotter, about 878 degrees Kelvin, or 1,120 degrees Fahrenheit.

It probably has a rocky core, wrapped in a thick, gaseous envelope of about 90 percent hydrogen. Its atmosphere is cloudless at high altitude, but as the team found, it contains the signature of water vapour.

UMD graduate student Jonathan Fraine, the paper's lead author, observed HAT P-11b using two NASA telescopes, the Hubble Space Telescope, which measures visible and near-infrared light, and the Spitzer Space Telescope, which records only infrared light, between July 2011 and December 2012.

The team compared those data to observations by NASA's Kepler Space Telescope, which was launched to look for exoplanets and continuously records images of the portion of the sky where HAT-P-11b is located.

Why do astronomers look for water on exoplanets? First, because water is a precondition for life – though the presence of water alone is not enough for life to arise. "The water molecule is widespread in the universe," says Deming.

"Wherever you have hydrogen and oxygen, it naturally forms. Even some sun spots are cool enough to contain water vapour, although obviously it's far too hot for life on the sun."

Astronomers also want to test the hypothesis that other planets formed the same way ours did. In the primordial solar system, particles of dust and ice carried native electrical charges that caused them to stick together, like household "dust bunnies" do, in a process called core accretion.

Early in this process the giant planets that formed far from the sun had enough gravitational pull to attract large amounts of hydrogen gas, the H in H2O.

But water freezes out of the atmospheres of our solar system's giant planets, where it occurs only at deep levels that are difficult to observe.

The closer-in, smaller planets, Mars, Venus and Earth, had water early in their evolution, though only Earth retains liquid water at the surface.

The smaller the planet, astronomers believe, the more likely it is that heavier molecules like water vapor will be abundant along with hydrogen.

"Our ideas about the formation of planets have been developed to match our solar system," explains Deming, "and we don't know whether other planetary systems behave the same way. We want to test the fundamental question of whether small planets are rich in heavy elements, like the oxygen in water vapour."

The finding of water vapor and hydrogen on HAT P-11b "is a key piece of the puzzle," Deming says, consistent with astronomers' main ideas on the formation of planets.

More information: "Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet," Jonathan Fraine, Drake Deming, Bjorn Benneke, Heather Knutson, Andrés Jordán, Néstor Espinoza, Nikku Madhusudhan, Ashlee Wilkins, and Kamen Todorov, was published in Nature on Sept. 25, 2014. dx.doi.org/10.1038/nature13785

40,000 People needed to Colonise any Alien Planet

Adrian Mann's illustration depicts a future starship under construction in Earth orbit using a ring-type construction facility, which could provide hotel rooms for guests who wish to view the construction.

Hungary-based space illustrator Adrian Mann is a graphical engineer for Project Icarus.
Credit: Adrian Mann

If humanity ever wants to colonise a planet beyond the solar system, it's going to need a really big spaceship.

The founding population of an interstellar colony should consist of 20,000 to 40,000 people, said Cameron Smith, an anthropologist at Portland State University in Oregon.

Such a large group would possess a great deal of genetic and demographic diversity, giving the settlement the best chance of survival during the long space voyage and beyond, he explained.

"Do you want to just squeak by, with barely what you can get? Or do you want to go in good health?" Smith said on July 16 during a presentation with NASA's Future In-Space Operations (FISO) working group (mp3 file).

"I would suggest, go with something that gives you a good margin for the case of disaster."

Revisiting the numbers
In the past, researchers have proposed that a few hundred people would be sufficient to establish a settlement on or near an alien planet but Smith thought it was time to take another look.

"I wanted to revisit the issue," he said. "It had been quite a long time, and of course we now know more about population genetics from genomics."

For his study, which was published in April in the journal Acta Astronautica, Smith assumed an interstellar voyage lasting roughly 150 years.

This time frame is consistent with that envisioned by researchers at Icarus Instellar, a nonprofit organization dedicated to pursuing travel to another star.

Smith's calculations, which combine information from population genetics theory and computer modeling, point toward a founding population of 14,000 to 44,000 people.

A "safe and well-considered figure" is 40,000, about 23,000 of whom would be men and women of reproductive age, Smith writes in the study.

This figure may seem "astoundingly large," Smith acknowledged, but he stressed that it makes sense.

Smith writes in the Acta Astronautica paper; "This number would maintain good health over five generations despite;

• increased inbreeding resulting from a relatively small human population, 
• depressed genetic diversity due to the founder effect, 
• demographic change through time and 
• expectation of at least one severe population catastrophe over the five-generation voyage,"

Data from the real world support the overall thrust of his findings, Smith added.

"Almost no natural populations of vertebrates dip below around five to 7,000 individuals," he said during the FISO talk.

" There are genetic reasons for this. And when they do go below this, sometimes they survive, but many times they go into what's called a demographic or extinction vortex."

Sending frozen sperm and eggs on the voyage with a limited number of human "tenders" is also an option, Smith said, though he didn't consider it seriously in the new paper.

"It can be done, but it's so different from the human experience of living in communities and so forth that I've kind of avoided that," he said.

"I'm kind of assuming, or sticking with, 'What is the experience of humanity so far, and what can we learn from it?"

Most precise measurement of an alien world's size: Kepler-93b

Using data from NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist's conception.

Credit: NASA/JPL-Caltech

The size of the exoplanet, dubbed Kepler-93b, is now known to an uncertainty of just 74 miles (119 kilometers) on either side of the planetary body.

The findings confirm Kepler-93b as a "super-Earth" that is about one-and-a-half times the size of our planet. Although super-Earths are common in the galaxy, none exist in our solar system.

Exoplanets like Kepler-93b are therefore our only laboratories to study this major class of planet.

With good limits on the sizes and masses of super-Earths, scientists can finally start to theorize about what makes up these weird worlds.

Previous measurements, by the Keck Observatory in Hawaii, had put Kepler-93b's mass at about 3.8 times that of Earth.

The density of Kepler-93b, derived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, like Earth.

"With Kepler and Spitzer, we've captured the most precise measurement to date of an alien planet's size, which is critical for understanding these far-off worlds," said Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of a paper on the findings published in the Astrophysical Journal.

"The measurement is so precise that it's literally like being able to measure the height of a six-foot tall person to within three quarters of an inch, if that person were standing on Jupiter," said Ballard.

Kepler-93b orbits a star located about 300 light-years away, with approximately 90 percent of the sun's mass and radius.

The exoplanet's orbital distance, only about one-sixth that of Mercury's from the sun—implies a scorching surface temperature around 1,400 degrees Fahrenheit (760 degrees Celsius).

Despite its new found similarities in composition to Earth, Kepler-93b is far too hot for life.

To make the key measurement about this toasty exoplanet's radius, the Kepler and Spitzer telescopes each watched Kepler-93b cross, or transit, the face of its star, eclipsing a tiny portion of starlight.

Kepler's unflinching gaze also simultaneously tracked the dimming of the star caused by seismic waves moving within its interior.

These readings encode precise information about the star's interior. The team leveraged them to narrowly gauge the star's radius, which is crucial for measuring the planetary radius.

Spitzer, meanwhile, confirmed that the exoplanet's transit looked the same in infrared light as in Kepler's visible-light observations.

These corroborating data from Spitzer, some of which were gathered in a new, precision observing mode, ruled out the possibility that Kepler's detection of the exoplanet was bogus, or a so-called false positive.

Taken together, the data boast an error bar of just one percent of the radius of Kepler-93b.

The measurements mean that the planet, estimated at about 11,700 miles (18,800 kilometers) in diameter, could be bigger or smaller by about 150 miles (240 kilometers), the approximate distance between Washington, D.C., and Philadelphia.

More Information: Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode - Authors: Sarah Ballard et al. 2014 ApJ 790 12. doi:10.1088/0004-637X/790/1/12

Search for extraterrestrial intelligence targeting alien polluters

In this artist's conception, the atmosphere of an Earth-like planet displays a brownish haze; the result of widespread pollution. 

New research shows that the upcoming James Webb Space Telescope (JWST) potentially could detect certain pollutants, specifically CFCs, in the atmospheres of Earth-sized planets orbiting white dwarf stars. 

Credit: Christine Pulliam (CfA)

Humanity is on the threshold of being able to detect signs of alien life on other worlds.

By studying exoplanet atmospheres, we can look for gases like oxygen and methane that only coexist if replenished by life but those gases come from simple life forms like microbes. What about advanced civilizations? Would they leave any detectable signs?

They might, if they spew industrial pollution into the atmosphere. New research by theorists at the Harvard-Smithsonian Center for Astrophysics (CfA) shows that we could spot the fingerprints of certain pollutants under ideal conditions. This would offer a new approach in the search for extraterrestrial intelligence (SETI).

"We consider industrial pollution as a sign of intelligent life, but perhaps civilizations more advanced than us, with their own SETI programs, will consider pollution as a sign of unintelligent life since it's not smart to contaminate your own air," says Harvard student and lead author Henry Lin.

"People often refer to ETs as 'little green men,' but the ETs detectable by this method should not be labeled 'green' since they are environmentally unfriendly," adds Harvard co-author Prof Avi Loeb.

The team, which also includes Smithsonian scientist Gonzalo Gonzalez Abad, finds that the upcoming James Webb Space Telescope (JWST) should be able to detect two kinds of chlorofluorocarbons (CFCs); ozone-destroying chemicals used in solvents and aerosols.

They calculated that JWST could tease out the signal of CFCs if atmospheric levels were 10 times those on Earth.

A particularly advanced civilization might intentionally pollute the atmosphere to high levels and globally warm a planet that is otherwise too cold for life.

There is one big caveat to this work. JWST can only detect pollutants on an Earth-like planet circling a white dwarf star, which is what remains when a star like our Sun dies.

That scenario would maximize the atmospheric signal. Finding pollution on an Earth-like planet orbiting a Sun-like star would require an instrument beyond JWST; a next-next-generation telescope.

The team notes that a white dwarf might be a better place to look for life than previously thought, since recent observations found planets in similar environments.

Those planets could have survived the bloating of a dying star during its red giant phase, or have formed from the material shed during the star's death throes.

While searching for CFCs could ferret out an existing alien civilization, it also could detect the remnants of a civilization that annihilated itself.

Some pollutants last for 50,000 years in Earth's atmosphere while others last only 10 years. Detecting molecules from the long-lived category but none in the short-lived category would show that the sources are gone.

"In that case, we could speculate that the aliens wised up and cleaned up their act. Or in a darker scenario, it would serve as a warning sign of the dangers of not being good stewards of our own planet," says Loeb.

SETI Kelper: Planet bonanza hints at worlds similar to our own

The artist concept depicts multiple-transiting planet systems, which are stars with more than one planet. 

The planets eclipse or transit their host star from the vantage point of the obplanetserver. This angle is called edge-on. 

Credit: NASA

For hunters, this has been a bountiful year. A team lead of astronomers at the SETI Institute and NASA Ames Research Center have used data from NASA's Kepler space telescope to uncover 715 new exoplanets.

The newly-verified objects orbit 305 different stars, and therefore include multi-world systems that are reminiscent of the Sun's own planetary family.

The announcement of these discoveries was followed by news that Kepler had also found the first Earth-size planet in the habitable zone of its star, Kepler 186f.

This is a significant milestone in the task of determining the prevalence of terrestrial planets in the Milky Way galaxy.

Jason Rowe

"These results are showing us that not only are Earth-sized planets common, but so are multi-planet systems containing potentially habitable worlds," notes Jason Rowe, a SETI Institute astronomer who co-led the study.

"Most of the new planets orbit their host star much closer than Mercury, but a few are beginning to bear a similarity to our own solar system."

The deluge of new planets has been intensified by a new analysis scheme called verification by multiplicity.

This technique can be applied to many planets at once, allowing the researchers to verify hundreds of new planetary systems in wholesale fashion, rather than teasing them from the Kepler data one-by-one as done in the past.

The new technique uses probability arguments based on the recognition that, of the 150,000 stars observed by Kepler, hundreds were found that have multiple planet candidates.

David Black

On this basis, the researchers are assured that their results are not distorted by binary stars that can mimic a multi-world system.

The new discoveries increase the total number of known exoplanets to over 1,700.

"From this work we've also learned that planets in these multiple systems are small, and their orbits are flat and circular, much like our own solar system," Rowe said.

On April 17th, the Kepler team announced the discovery of Kepler 186f, the first Earth-sized planet found in the habitable zone of its host star, marking a major milestone in determining the frequency of Earth-like planets in the Milky Way galaxy.

"Uncovering these worlds and showing that habitable worlds could be very common has increased the likelihood that there is life, perhaps abundant life, elsewhere in the cosmos," notes David Black, President and CEO of the SETI Institute.

Data collection from the Kepler mission ended in the spring of last year, due to the failure of a second on-board reaction wheel, essential to accurate pointing of the telescope.

However, on May 20th, NASA announced the approval of the K2 mission, intended to repurpose Kepler to use the pressure of sunlight hitting the side of the spacecraft to act as a third wheel.

"We can't continue to look at the original Kepler star field," said Douglas Caldwell, Kepler Instrument Scientist at the SETI Institute, "but spacecraft are built and operated by very smart people, and thanks to the hard work of the entire Kepler team we can now search for planets in a wide variety of environments and conditions, including star forming regions. Doing so will teach us more about how our own planetary system formed and evolved."

"The more we explore the more we find worlds among the stars that remind us of home," Rowe notes.

More information: Jason Rowe is presenting these results at this week's annual meeting of the Canadian Astronomical Society (CASCA) in Quebec: casca2014.craq-astro.ca/index_en.php

NASA Kepler: 'Neapolitan' exoplanets come in three flavours

This artist's conception shows a planet forming from a disk of gas and dust surrounding a young star. 

Credit: David A. Aguilar (CfA)

The planets of our solar system come in two basic flavors, like vanilla and chocolate ice cream.

We have small, rocky terrestrials like Earth and Mars, and large gas giants like Neptune and Jupiter.

We're missing the astronomical equivalent of strawberry ice cream - planets between about one and four times the size of Earth. NASA's Kepler mission has discovered that these types of planets are very common around other stars.

New research following up on the Kepler discoveries shows that alien worlds, or exoplanets, can be divided into three groups, terrestrials, gas giants, and mid-sized "gas dwarfs", based on how their host stars tend to fall into three distinct groups defined by their compositions.

"We were particularly interested in probing the planetary regime smaller than four times the size of Earth, because it includes three-fourths of the planets found by Kepler."

"That's where you'll find rocky worlds, which are the only kind that we would consider potentially habitable," says lead author Lars A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA).

Buchhave presented his research today in a press conference at a meeting of the American Astronomical Society.

Kepler finds exoplanets using the transit method, looking for a star that dims as a planet passes in front of it from our point of view.

We can learn the planet's size from how much starlight it blocks. However, to determine the planet's composition we need to measure its mass, so its density can be calculated.

A rocky planet will be much denser than a gas giant. Unfortunately, the smaller a planet, the harder it is to measure its mass, especially for the dim and distant stars examined by Kepler.

New research finds that exoplanets can be divided into three groups, terrestrials, gas giants, and mid-sized "gas dwarfs," based on how their host stars tend to fall into three distinct groups defined by their compositions. 

All three are portrayed in this artist's conception. 

Credit: J. Jauch

Buchhave and his colleagues took a different approach. They measured the amount of elements heavier than hydrogen and helium, which astronomers collectively call metals, in stars with exoplanet candidates.

Since a star and its planets form from the same disk of material, the metallicity of a star reflects the composition of the protoplanetary disk.

The team took follow-up spectra of more than 400 stars hosting over 600 exoplanets.

Then, they conducted a statistical test to see if the sizes of the planets fell into natural groups, along with the stellar metallicities.

They found two clear dividing lines - one at a size 1.7 times as large as Earth and the other at a size 3.9 times larger than Earth.

They infer that these boundaries also mark changes in composition. Planets smaller than 1.7 Earths are likely to be completely rocky, while those larger than 3.9 Earths are probably gas giants.

More information: Research Paper: v509/n7502/full/nature13254.html

Scanning the skies for Exoplanets and Exomoons in other solar systems

The best prospect for habitable exomoons may be around gas giants. 

Credit: NASA

The first exoplanet was discovered in 1994.

Twenty years later, NASA's exoplanet catalog lists more than 1700 planets confirmed around other stars.

Most of these extra-solar-systems have been measured by changes in light spectra, in stellar motion or dust disks around stars.

Some exoplanets-more than 40 as of today-have even been directly photographed.

Jupiter's moons

One way or the other, we know that exoplanets are out there in abundance, in places we thought they would be and in places we didn't dream a planet could possibly exist. So what comes next? Finding moons.

Exomoons are naturally formed satellites circling around planets in other solar systems. Like the exoplanets themselves, we assume that exomoons are out there in relatively high abundance.

This assumption is based partly upon what we see around us in our own Solar System and partly upon our hypotheses about planetary formation.

Saturn's moons

This is what we observe in our own Solar System: moons are extremely common.

From Earth's one Moon to Jupiter's (currently known) fifty, every planet in the Solar System one astronomical unit or more from the Sun has a natural satellite.

Even Pluto, no longer officially classified as a planet, has a smaller companion circling around it.

Of note, the solid bodies such as Earth and Pluto have very few companions, while gaseous bodies Jupiter, Saturn, Uranus and Neptune have many.

Pluto and Charon

Furthermore, the masses of the Moon and Charon have a very specific relationship to Earth and Pluto in terms of mass: each satellite is about 10-2 the mass of their parent planet.

By contrast, the ratio of satellite masses to parent planet masses for the gas giants is very different: 10-4.

The differences in mass-ratio, how massive the moon is compared to the parent planet, and the differences in composition between the moons of solid planets and those of the gas giants led to a search for different formation scenarios for Earth's moon and the moons of the outer planets.

This is the current hypothesis: that there are two different methods of satellite formation at work in our Solar System.

Amy Barr Mlinar

Both methods were recently reviewed by Dr.Amy Barr Mlinar of Brown University at the Space Telescope Science institute Spring Symposium.

"This has been worked out starting about in the 1960's up through now," said Barr, "You have this [moon/planet] mass ratio of about 10-2 for solid planets, and a [moon/planet] mass ratio of about 10-4 for planets with a gaseous envelope."

Essentially, difference in mass ratios reflects the two completely different origins of our Moon and the satellites of Jupiter.

At the high end of the moon/planet mass ratio, 10-2 are the satellites of solid bodies (Earth and Pluto). These moons were formed from collisions.

Sometime in the distant past an object some large percentage of Earth's size struck the Earth, knocking material away that later coalesced into the Moon. The same is likely true of Charon, Pluto's companion.

Read the full article here

NASA Cassini: Sunsets on Titan reveal the complexity of hazy exoplanets - Video

Using data collected by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) while observing Titan's sunsets, researchers created simulated spectra of Titan as if it were a planet transiting across the face of a distant star. 

The research helps scientists to better understand observations of exoplanets with hazy atmospheres. 

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 

JPL manages the mission for NASA's Science Mission Directorate, Washington. 

The California Institute of Technology (CalTech) in Pasadena manages JPL for NASA. The VIMS team is based at the University of Arizona in Tucson. 

Credit: NASA/JPL-Caltech

Scientists working with data from NASA's Cassini-Huygens mission have developed a new way to understand the atmospheres of exoplanets by using Saturn's smog-enshrouded moon Titan as a stand-in.

The new technique shows the dramatic influence that hazy skies could have on our ability to learn about these alien worlds orbiting distant stars.

Tyler Robinson

The work was performed by a team of researchers led by Tyler Robinson, a NASA Postdoctoral Research Fellow at NASA's Ames Research Center in Moffett Field, California.

The findings were published May 26 in the Proceedings of the National Academy of Sciences.

"It turns out there's a lot you can learn from looking at a sunset," Robinson said.

Light from sunsets, stars and planets can be separated into its component colors to create spectra, as prisms do with sunlight, in order to obtain hidden information.

Despite the staggering distances to other planetary systems, in recent years researchers have begun to develop techniques for collecting spectra of exoplanets.

When one of these worlds transits, or passes in front of its host star as seen from Earth, some of the star's light travels through the exoplanet's atmosphere, where it is changed in subtle, but measurable, ways.

This process imprints information about the planet that can be collected by telescopes. The resulting spectra are a record of that imprint.

Spectra enable scientists to tease out details about what exoplanets are like, such as aspects of the temperature, composition and structure of their atmospheres.

Tyler Robinson and his colleagues exploited a similarity between exoplanet transits and sunsets witnessed by the Cassini spacecraft at Titan.

These observations, called solar occultations, effectively allowed the scientists to observe Titan as a transiting exoplanet without having to leave the solar system. In the process, Titan's sunsets revealed just how dramatic the effects of hazes can be.

Multiple worlds in our own solar system, including Titan, are blanketed by clouds and high-altitude hazes. Scientists expect that many exoplanets would be similarly obscured.

Clouds and hazes create a variety of complicated effects that researchers must work to disentangle from the signature of these alien atmospheres, and thus present a major obstacle for understanding transit observations.

Due to the complexity and computing power required to address hazes, models used to understand exoplanet spectra usually simplify their effects.

"Previously, it was unclear exactly how hazes were affecting observations of transiting exoplanets," said Robinson. "So we turned to Titan, a hazy world in our own solar system that has been extensively studied by Cassini."

With Titan as their example, Robinson and colleagues found that hazes high above some transiting exoplanets might strictly limit what their spectra can reveal to planet transit observers.

The observations might be able to glean information only from a planet's upper atmosphere. On Titan, that corresponds to about 90 to 190 miles (150 to 300 kilometers) above the moon's surface, high above the bulk of its dense and complex atmosphere.

An additional finding from the study is that Titan's hazes more strongly affect shorter wavelengths, or bluer, colors of light.

Studies of exoplanet spectra have commonly assumed that hazes would affect all colours of light in similar ways. Studying sunsets through Titan's hazes has revealed that this is not the case.

Mark Marley

"People had dreamed up rules for how planets would behave when seen in transit, but Titan didn't get the memo," said Mark Marley, a co-author of the study at NASA Ames.

"It looks nothing like some of the previous suggestions, and it's because of the haze."

The team's technique applies equally well to similar observations taken from orbit around any world, not just Titan.

This means that researchers could study the atmospheres of planets like Mars and Saturn in the context of exoplanet atmospheres as well.

Curt Niebur

"It's rewarding to see that Cassini's study of the solar system is helping us to better understand other solar systems as well," said Curt Niebur, Cassini program scientist at NASA Headquarters in Washington.

More information: Titan solar-occultation observations reveal transit spectra of a hazy world, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1403473111

Leaving Exoplanets aside, Exomoons may harbour life too

Shooting for the exomoon. 

Credit: CBC11, CC BY-SA

In the Star Wars universe, everyone's favourite furry aliens, the Ewoks, famously lived on the "forest moon of Endor".

In scientific terms, the Ewok's home world would be referred to as an exomoon, which is simply a moon that orbits an exoplanet, any planet that orbits a star other than our sun.

Although more than 1,000 exoplanets have been discovered since the first one was found in 1995, only a handful of those are thought to be habitable, at least by life as we know it.

New research shows that exomoons, too, could provide habitable environments. Although we are yet to find exomoons, we have good reasons to believe that there should be many, even more than exoplanets.

Goldilocks zone
Perhaps the most habitable planet found to date is the recently announced Kepler-186f.

This is one of five exoplanets discovered by NASA's Kepler satellite, all orbiting a small, faint, red dwarf star, 500 light years away in the constellation of Cygnus.

Kepler-186f is an Earth-sized planet that orbits its star in only 130 days and is about as distant from its star as Mercury is from the Sun. But, because the red dwarf is much dimmer than the Sun, Kepler-186f receives only about one-third of the energy that the Earth does.

As a result, Kepler-186f lies at the outer edge of its star's "habitable zone".

This is the hypothetical region of space surrounding a star in which liquid water may conceivably exist on the surface of any exoplanets.

In our own solar system, Venus lies too close to the Sun and is too hot.

Mars lies too far from the Sun and is too cold.

But Earth, of course, lies within the critical "Goldilocks zone", where the temperature is just right.

Simply residing in the habitable zone, though, is no guarantee that an exoplanet has water oceans.

The climate of a planet is much more complicated than we can capture with a simple calculation based on the distance of a planet from a star.

We know that Mars probably had running water on its surface in the past, but now it is a frozen desert. Earth, meanwhile, was probably in a completely frozen "snowball" state about 650m years ago.

Duncan Forgan

Recent research by Duncan Forgan and Vergil Yotov at the University of Edinburgh highlights the various factors that may make an exomoon more or less habitable.

They investigate how the climate of an exomoon will be affected by tidal stresses which provide a source of internal heating for the exomoon as it is stretched and deformed by the gravitational pull of its planet.

They also investigated how light reflected from the exoplanet, and eclipses by the exoplanet, can also subtly alter the exomoon's climate.

The researchers lump theoretical exomoons into a number of classifications: "habitable", "hot", "snowball" or "transient".

Those in the first class have more than 10% of their surface at a temperature between the freezing and boiling points of water, with only a small fluctuation around the average temperature value.

Those in the second class have average temperatures above 100°C at all times, whereas those in the third class are permanently frozen, in both cases less than 10% of the surface is habitable.

Exomoons in the fourth, transient class are on average habitable, but the amount of habitable surface area varies widely with time.

Overall, this research shows that exomoon climates are rather more complex than previous research has supposed.

As yet, no exomoons have been discovered, but there are various techniques proposed for finding them.

One way is by studying the effects that an exomoon will have on the exoplanet it is orbiting – their gravitational connection means there will be a to-and-fro tugging between them.

This will cause variations in the times at which the planet transits in front of its star and in the durations of these transits, which we are able to measure.

These time variations will only be a few seconds at most, so very accurate measurements of the transits must be made to reveal the exomoon's presence.

If variations are detected then, in principle, both the mass and orbit of the exomoon may be calculated from the measurements.

It surely is only a matter of time before the first exomoon is discovered and the probability of finding one in the habitable zone of a star is reasonably high.

We may not find any Ewoks, but habitable exomoons may indeed offer the best prospect for hosting alien life.

Gemini South Telescope: A new eye to scan the sky for exoplanets

The Gemini South telescope houses the latest gear to hunt down and snap photos of exoplanets. 

Credit: Gemini Observatory, CC BY 

There is excitement in astronomy and planetary science departments worldwide as the new Gemini Planet Imager (GPI), housed in the Gemini South Telescope in the Chilean Andes, turns its razor-sharp gaze to the skies.

This device, known as GPI for short, is the first of a small handful of sophisticated instruments to attempt a task that until recently was considered all but impossible: to image the faint mote of light betraying the presence of a planet nestled against the overwhelming glare of its host star.

Planets in orbit around distant stars, exoplanets, are now known to adorn more than 1,000 star systems. There is possibly five times that number under strong suspicion awaiting only final confirmatory data to join the club.

You could be forgiven for thinking this avalanche of discovery – all coming in the past 20 years – has settled most of the important questions in exoplanetary science.

The reality, though, is it hasn't.


Location, location, location
The sample of exoplanets we now have tells us far more about the limitations of the techniques we use to find them than it does about the exoplanets themselves. We have only seen the tip of the iceberg.

The search can be likened to the proverbial scientist in a dark car park searching for a set of dropped car keys under the only streetlight.

A passer-by asks: "Did you drop your keys there?" "No," you reply. "I dropped them somewhere over there in the dark, but I can only see here."

That patch of discovery illuminated by our present instruments particularly favours the largest planets in the closest orbits about their host stars.

The extreme examples of this (and the most celebrated exoplanet discovery, of 51 Peg, that launched the field in 1995) are known as "hot Jupiters".

The name understates their inhospitable crushing gravity combined with searing radiation field from the looming host star.

In a quest to identify planets capable of supporting life hot Jupiters score low. Astronomers are working on a valuation scheme that would identify those that lie within the so-called "habitable zone".

ESA PLATO: 34 European space telescopes will investigate one million stars

This is what the planet-hunter PLATO could look like. 

The image shows a concept presented by Thales Alenia Space. 

Credit: ESA

The exploration of planets around stars other than the Sun, known as extrasolar planets or 'exoplanets', is one of the most exciting topics of 21st century science.

One of the key goals of this research is to discover and learn the properties of Earth-like worlds in the Sun's neighbourhood.

ESA, the European Space Agency, will do this in preparing a new space mission named PLATO.

The mission's launch is scheduled for 2024, and firm discoveries of Earth-like planets at Earth-like distances from stars similar to our Sun will be produced after three years of observational data have been collected.

ESA's Science Programme Committee voted for PLATO at its regular meeting in Paris on 19th and 20th February, 2014, where it was one of five proposed space projects for a so-called "M" mission.

Not a single Earth-like exoplanet in a habitable zone around a star similar to our Sun has been found and characterized yet. PLATO will be a pioneer in finding new worlds for humanity to explore.

The PLATO Science consortium is led by Don Pollacco from the University of Warwick who comments

"This is fantastic news for Europe, PLATO will allow the first systematic survey of nearby planets for indications from advanced life forms (as well as slime)."

"A few years ago this would have been science fiction and now its coming to pass as science fact."

The UK also has major roles in the instrument itself supplying the CCD sensors (e2V Technology and UCL), much of the image processing software (Cambridge) and Public Outreach (Open University).

Heike Rauer

The PLATO mission itself is led by Dr Heike Rauer at DLR, the German Aerospace Center. "PLATO will begin a completely new chapter in the exploration of extrasolar Planets" Dr Rauer confidently predicts.

"We will find planets that orbit their star in the life-sustaining 'habitable' zone: planets where liquid water is expected, and where life as we know it can be maintained." she continued.

PLATO will measure the sizes, masses, and ages of the planetary systems it finds, so detailed comparisons with our own Solar System can be made.

"In the last 20 years more than one thousand exoplanets have been discovered, with quite a few multi-planetary systems among them", Rauer explains.

"But almost all of these systems differ significantly from our Solar System in their properties, because they are the easiest-to-find examples."

"PLATO firmly will establish whether systems like our own Solar System, and planets like our own Earth are common in the Galaxy."

This is a concept image of Plato by EADS Astrium, ESA's preferred supplier for space technology. 

PLATO, is an acronym for PLanetary Transits and Oscillations of Stars.

PLATO will find planets through the periodic dimming of the detected starlight caused by a planet orbiting in front of the star, blocking PLATO's view of a fraction of the starlight.

PLATO will also measure tiny changes in detected starlight caused by small vibrations in the host stars, performing so-called astroseismology.

Just as in seismology of the Earth, these vibrations reveal the interior structure of the vibrating body. Astroseismology allows us to learn the age of the vibrating star and the planets orbiting around it.

A new type of space telescope
ESA's PLATO is a completely new type of space telescope: it will use an array of telescopes rather than a single lens or mirror.

PLATO will use top quality cameras, and will have the advantage of observing continuously from space, without the interruption of sunrise, or the blurring caused by the Earth's atmosphere.

This will allow PLATO to discover planets smaller than Earth, and planets at distances from their host stars similar to the Earth-Sun distance.

So far, only a few small exoplanets are known at star-planet distances comparable to or greater than Earth's.

Unlike previous missions, PLATO will focus on these planets, which are expected to resemble our own Solar System planets.

European Southern Observatory's European Extremely Large Telescope (E-ELT)

ESA will take on a leading role in the search for extrasolar planets

PLATO is a lively and vigorous European collaboration, many European research institutions and hundreds of European researchers are working together, with scientists from all over the world completing the team.

The catalogue of potentially habitable planets provided by PLATO will be the basis for follow-up measurements to confirm discoveries of new planets, using the ESO's E-ELT, or the next generation of large space telescopes, like NASA's James Webb Space Telescope.

With PLATO, Europe will be leading the search for habitable exoplanets.

So far only a few exoplanets have had their mass, radius and age determined precisely. This is needed to properly describe a planet.

"The observation of planets in many different states of their evolution will give us clues for the past and the future of our own planetary system", Dr Rauer remarked.

"By no means do we know all about the youth of our Solar System."

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