CFA Astronomers find a new type of planet: The 'mega-Earth'

The newly discovered 'mega-Earth' Kepler-10c dominates the foreground in this artist's conception. 

Its sibling, the lava world Kepler-10b, is in the background. 

Both orbit a sunlike star. Kepler-10c has a diameter of about 18,000 miles, 2.3 times as large as Earth, and weighs 17 times as much. 

Therefore it is all solids, although it may possess a thin atmosphere shown here as wispy clouds. 

Credit: David A. Aguilar (CfA)

Astronomers announced today that they have discovered a new type of planet - a rocky world weighing 17 times as much as Earth.

Theorists believed such a world couldn't form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant.

This planet, though, is all solids and much bigger than previously discovered "super-Earths," making it a "mega-Earth."

"We were very surprised when we realized what we had found," says astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the data analysis and made the discovery.

"This is the Godzilla of Earths!" adds CfA researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative. "But unlike the movie monster, Kepler-10c has positive implications for life."

The team's finding was presented today in a press conference at a meeting of the American Astronomical Society (AAS).

The newfound mega-Earth, Kepler-10c, circles a sunlike star once every 45 days. It is located about 560 light-years from Earth in the constellation Draco. The system also hosts a 3-Earth-mass "lava world," Kepler-10b, in a remarkably fast, 20-hour orbit.

Kepler-10c was originally spotted by NASA's Kepler spacecraft. Kepler finds planets using the transit method, looking for a star that dims when a planet passes in front of it.

By measuring the amount of dimming, astronomers can calculate the planet's physical size or diameter. However, Kepler can't tell whether a planet is rocky or gassy.

Kepler-10c was known to have a diameter of about 18,000 miles, 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.

The team used the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to measure the mass of Kepler-10c.

They found that it weighed 17 times as much as Earth - far more than expected. This showed that Kepler-10c must have a dense composition of rocks and other solids.

"Kepler-10c didn't lose its atmosphere over time. It's massive enough to have held onto one if it ever had it," explains Dumusque. "It must have formed the way we see it now."

Planet formation theories have a difficult time explaining how such a large, rocky world could develop. However, a new observational study suggests that it is not alone.

Also presenting at AAS, CfA astronomer Lars A. Buchhave found a correlation between the period of a planet (how long it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous.

This suggests that more mega-Earths will be found as planet hunters extend their data to longer-period orbits.

The discovery that Kepler-10c is a mega-Earth also has profound implications for the history of the universe and the possibility of life.

The Kepler-10 system is about 11 billion years old, which means it formed less than 3 billion years after the Big Bang.

"Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life," says Sasselov.

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

ILLUSTRIS: Astronomers create first realistic virtual universe

Move over, Matrix - astronomers have done you one better. They have created the first realistic virtual universe using a computer simulation called "Illustris."

Illustris can recreate 13 billion years of cosmic evolution in a cube 350 million light-years on a side with unprecedented resolution.

Mark Vogelsberger

"Until now, no single simulation was able to reproduce the universe on both large and small scales simultaneously," says lead author Mark Vogelsberger (MIT/Harvard-Smithsonian Center for Astrophysics), who conducted the work in collaboration with researchers at several institutions, including the Heidelberg Institute for Theoretical Studies in Germany.

These results are being reported in the May 8th issue of the journal Nature.

Previous attempts to simulate the universe were hampered by lack of computing power and the complexities of the underlying physics.

As a result those programs either were limited in resolution, or forced to focus on a small portion of the universe.

Earlier simulations also had trouble modeling complex feedback from star formation, supernova explosions, and supermassive black holes.

Illustris employs a sophisticated computer program to recreate the evolution of the universe in high fidelity. It includes both normal matter and dark matter using 12 billion 3-D "pixels," or resolution elements.

Large scale projection through the Illustris volume at z=0, centered on the most massive cluster, 15 Mpc/h deep. 

Shows dark matter density (left) transitioning to gas density (right). 

Credit: Illustris Collaboration

The team dedicated five years to developing the Illustris program.

The actual calculations took 3 months of "run time," using a total of 8,000 CPUs running in parallel.

If they had used an average desktop computer, the calculations would have taken more than 2,000 years to complete.

The computer simulation began a mere 12 million years after the Big Bang. When it reached the present day, astronomers counted more than 41,000 galaxies in the cube of simulated space.

Importantly, Illustris yielded a realistic mix of spiral galaxies like the Milky Way and football-shaped elliptical galaxies.

It also recreated large-scale structures like galaxy clusters and the bubbles and voids of the cosmic web. On the small scale, it accurately recreated the chemistries of individual galaxies.

Large scale projection through the Illustris volume at z=0, centered on the most massive cluster, 15 Mpc/h deep. 

Shows dark matter density overlaid with the gas velocity field. 

Credit: Illustris Collaboration

Since light travels at a fixed speed, the farther away astronomers look, the farther back in time they can see.

A galaxy one billion light-years away is seen as it was a billion years ago.

Telescopes like Hubble can give us views of the early universe by looking to greater distances.

However, astronomers can't use Hubble to follow the evolution of a single galaxy over time.

"Illustris is like a time machine. We can go forward and backward in time. We can pause the simulation and zoom into a single galaxy or galaxy cluster to see what's really going on," says co-author Shy Genel of the CfA.

The team is releasing a high-definition video, which morphs between different components of the simulation to highlight various layers (e.g. dark matter density, gas temperature, or chemistry).

They also are releasing several smaller videos and associated imagery online

More information: Paper: dx.doi.org/10.1038/nature13316

Rumours that Gravitational waves have been detected

This detailed map of the Cosmic Microwave Background (CMB) is created from seven years worth of data. 

The colour variations correspond to temperature variations in the young universe: the seeds for stars and galaxies observed today. 

Credit: NASA

Last week the Harvard-Smithsonian Center for Astrophysics (CfA) stated rather nonchalantly that they will be hosting a press conference on Monday, March 17th, to announce a "major discovery."

Without a potential topic for journalists to muse on, this was as melodramatic as it got but then the Guardian posted an article on the subject and the rumours went into overdrive.

The speculation is this: a U.S. team is on the verge of confirming they have detected primordial gravitational waves—ripples in the fabric of spacetime that carry echoes of the big bang nearly 14 billion years ago.

If there is evidence for gravitational waves, it will be a landmark discovery, ultimately changing the face of physics.

Not only are gravitational waves the last untested prediction of Albert Einstein's General Theory of Relativity, but primordial gravitational waves will allow astronomers to glimpse the universe in its infancy.

"It's been called the Holy Grail of cosmology," Hiranya Peiris, a cosmologist from University College London, told reporters.

"It would be a real major, major, major discovery." Any convincing evidence would almost certainly lead to a Nobel prize.

The signal is rumoured to have been found by a telescope known as BICEP (Background Imaging of Cosmic Extragalactic Polarization), which scans the sky from the south pole, looking for a subtle effect in the Cosmic Microwave Background (CMB): the radiation released 380,000 years after the big bang when space became transparent to light and photons were allowed to travel freely across the universe.

The South Pole Telescope (left) and BICEP (right). Credit: Dana Hrubes

While the CMB has been mapped in exquisite detail, astronomers think that hidden within the map is a second fingerprint, which would reveal gravitational waves.

Its radiation was scattered toward us from the universe's earliest atoms, similar to the way blue light is scattered toward us from the atoms in the sky and just as the sky is slightly polarized, the waves have a preferred orientation, so is the CMB (on the level of a few percent).

Cosmologists are digging through the data, searching for a subtle twist in the polarized light, known as B-modes.

If a gravitational wave moves through the fabric of spacetime, it will squeeze spacetime in one direction (the universe will look a little hotter) and stretch it in another (the universe will look a little cooler).

The photons will scatter with a preferred direction, leaving a slightly polarized imprint on the CMB, due to the passing gravitational wave.

Andrew Jaffe

"If a detection has been made, it is extraordinarily exciting," Andrew Jaffe, a cosmologist from Imperial College, London, told reporters.

"This is the real big tick-box that we have been waiting for. It will tell us something incredibly fundamental about what was happening when the universe was only 10-34 seconds old."

Scientists to Unveil 'Major Discovery' at Harvard Astrophysics Center

This illustration summarizes the almost 14-billion-year-long history of our universe. 

It shows the main events that occurred between the initial phase of the cosmos, where its properties were almost uniform and punctuated only by tiny fluctuations, to the rich variety of cosmic structure that we observe today, ranging from stars and planets to galaxies and galaxy clusters. 

Credit: ESA and the Planck Collaboration

A team of scientists will unveil what they bill as a "major discovery" in the field of astrophysics on Monday (March 17) in a presentation at the Harvard-Smithsonian Center for Astrophysics.

CfA officials did not detail the nature of the astrophysics discovery in a media alert. They only stated that the center will "host a press conference at 12:00 noon EDT (16:00 UTC) on Monday, March 17th, to announce a major discovery."

The Harvard-Smithsonian Center for Astrophysics is made up of the Harvard College Observatory and the Smithsonian Astrophysics Observatory.

Scientists at the center pursue studies of those basic physical process that determine the nature and evolution of the universe," according to the CfA website's official description.

Herschel: SMA unveils how small cosmic seeds in Snake nebula grow into big stars

These two panels show the Snake nebula as photographed by the Spitzer and Herschel space telescopes. 

At mid-infrared wavelengths (the upper panel taken by Spitzer), the thick nebular material blocks light from more distant stars. 

At far-infrared wavelengths, however (the lower panel taken by Herschel), the nebula glows due to emission from cold dust. 

The two boxed regions, P1 and P6, were examined in more detail by the Submillimeter Array. 

Credit: Spitzer /GLIMPSE /MIPS, Herschel /HiGal, Ke Wang, European Southern Observatory

New images from the Smithsonian's Submillimeter Array (SMA) telescope provide the most detailed view yet of stellar nurseries within the Snake nebula.

These images offer new insights into how cosmic seeds can grow into massive stars.

Stretching across almost 100 light-years of space, the Snake nebula is located about 11,700 light-years from Earth in the direction of the constellation Ophiuchus.

In images from NASA's Spitzer Space Telescope it appears as a sinuous, dark tendril against the starry background. It was targeted because it shows the potential to form many massive stars (stars heavier than 8 times our Sun).

"To learn how stars form, we have to catch them in their earliest phases, while they're still deeply embedded in clouds of gas and dust, and the SMA is an excellent telescope to do so," explained lead author Ke Wang of the European Southern Observatory (ESO), who started the research as a predoctoral fellow at the Harvard-Smithsonian Center for Astrophysics (CfA).

The team studied two specific spots within the Snake nebula, designated P1 and P6. Within those two regions they detected a total of 23 cosmic "seeds" - faintly glowing spots that will eventually birth one or a few stars.

The seeds generally weigh between 5 and 25 times the mass of the Sun, and each spans only a few thousand astronomical units (the average Earth-Sun distance).

The sensitive, high-resolution SMA images not only unveil the small seeds, but also differentiate them in age.

Previous theories proposed that high-mass stars form within very massive, isolated "cores" weighing at least 100 times the mass of the Sun.

These new results show that that is not the case. The data also demonstrate that massive stars aren't born alone but in groups.

"High-mass stars form in villages," said co-author Qizhou Zhang of the CfA. "It's a family affair."

The team also was surprised to find that these two nebular patches had fragmented into individual star seeds so early in the star formation process.

They detected bipolar outflows and other signs of active, ongoing star formation. Eventually, the Snake nebula will dissolve and shine as a chain of several star clusters.

NASA Asteroid Robotic Retrieval Mission: Retrieving an asteroid - Tempel1

An image of the asteroid Tempel 1 taken during the Deep Impact visit. 

Tempel 1 is about five kilometers across. 

CfA astronomers have estimated the size of the smallest measured near Earth asteroid, 2009 BD, as only about three meters across, perhaps too small for it to be useful in NASA's planned asteroid recovery mission. 

Credit: NASA/JPL-Caltech/UMd

Asteroids (or comets) whose orbits bring them close to the earth's orbit are called near Earth objects.

Some of them are old, dating from the origins of the solar system about four and one-half billion years ago, and expected to be rich in primitive materials.

They are of great interest to scientists studying the young solar system. Others, of lower scientific priority, are thought to contain minerals of potential economic value.

NASA has announced its interest in sending a manned mission to a near Earth object. The NASA Asteroid Robotic Retrieval Mission concept involves the capture of an asteroid, and dragging it onto a new trajectory that traps it in the Earth–Moon system where it will be further investigated by astronauts.

The current mission design requires the target asteroid to have a diameter of seven to ten meters. The object NEO 2009BD is a prime candidate for this retrieval mission.

It was discovered on January 16, 2009, at a distance from the Earth of only 0.008 AU (one AU is the average distance of the Earth from the Sun).

Its orbit is very Earth–like, with a period of 400 days, and it will end up close to the Earth–Moon system again in late 2022 when the proposed capture would take place.

It seems to be a perfect candidate, with a time frame that allows for proper mission planning.

The problem is that the size of the NEO 2009BD is uncertain, and thus its density and composition are also uncertain, but first estimates are that it likely falls in the diameter range specified by the mission.

The uncertainty arises because it was detected at optical wavelengths; they measure reflected light, which is a combination of both an object's size and reflectivity (albedo).

For NASA mission planning to succeed, a more direct size measurement of 2009 BD is needed—and soon, before its increasing distance from the Earth makes such an observation a practical impossibility.

CfA astronomers Joe Hora, Howard Smith and Giovanni Fazio have been regularly using the IRAC camera on the Spitzer Space Telescope to measure the infrared emission of near Earth objects, and (with some modeling) deriving both the sizes and densities of these objects.

They received special observatory time to study NEO 2009BD, and in an upcoming issue of the Astrophysical Journal they and their colleagues report on their conclusions.

They did not detect the NEO 2009BD to a very low light level, implying that it is very small, probably only about 2.9 meters in diameter, and modeling suggests it has a rubble-pile composition.

This is the smallest object ever reported on by Spitzer; whether it is still suitable for a NASA mission is now something that the NASA Retrieval Mission team must determine.

More information: "Constraining the Physical Properties of The Near–Earth Object 2009 BD," M. Mommert,J. L. Hora,D. E. Trilling,S. R. Chesley and D. Farnocchia,D. Vokrouhlick´y, M. Mueller,A. W. Harris, H. A. Smith and G. G. Fazio, ApJ, 2013, in press.

SMA reveals giant star cluster in the making

This image from the Smithsonian's Submillimeter Array maps the projected density of molecular gas in the central 30 light years of W49A. 

Brighter colours mark denser regions. 

The brightest region at the image center is less than three light-years across, yet it contains about 50,000 suns' worth of molecular gas. 

Credit: Roberto Galván-Madrid (ESO), Hauyu Baobab Liu (ASIAA, Taiwan), Tzu-Cheng Peng (ESO)

W49A might be one of the best-kept secrets in our galaxy.

This star-forming region shines 100 times brighter than the Orion nebula, but is so obscured by dust that very little visible or infrared light escapes.

The Smithsonian's Submillimeter Array (SMA) has peered through the dusty fog to provide the first clear view of this stellar nursery. The SMA revealed an active site of star formation being fed by streamers of infalling gas.

"We were amazed by all the features we saw in the SMA images," says lead author Roberto Galván-Madrid, who conducted this research at the Harvard-Smithsonian Center for Astrophysics (CfA) and the European Southern Observatory (ESO).

W49A is located about 36,000 light-years from Earth, on the opposite side of the Milky Way.

It represents a nearby example of the sort of vigorous star formation seen in so-called "starburst" galaxies, where stars form 100 times faster than in our galaxy.

The heart of W49A holds a giant yet surprisingly compact star cluster. About 100,000 stars already exist within a space only 10 light-years on a side.

In contrast, fewer than 10 stars lie within 10 light-years of our Sun. In a few million years, the giant star cluster in W49A will be almost as crowded as a globular cluster.

The SMA also revealed an intricate network of filaments feeding gas into the center, much like tributaries feed water into mighty rivers on Earth.

Being denser than average will help the W49A star cluster to survive. Most star clusters in the galactic disk dissolve rapidly, migrating away from each other under the influence of gravitational tides.

This is why none of the Sun's sibling stars remain nearby. Since it is so compact, the cluster in W49A might remain intact for billions of years.

The Submillimeter Array mapped the molecular gas within W49A in exquisite detail.

It showed that central 30 light-years of W49A is several hundred times denser than the average molecular cloud in the Milky Way.

In total, the nebula contains about 1 million suns' worth of gas, mostly molecular hydrogen.

Black Holes Have Simple Feeding Habits

At the centre of spiral galaxy M81 is a supermassive black hole about 70 million times more massive than our sun.

Image Credit: X-ray: NASA /CXC /Wisconsin /D.Pooley & CfA /A.Zezas; Optical: NASA /ESA /CfA /A.Zezas; UV: NASA /JPL-Caltech /CfA /J.Huchra et al.; IR: NASA /JPL-Caltech /CfA

Runaway binary stars

A Hubble image of the binary stars Sirius A and B. Sirius B, the very faint companion, is a white dwarf, an evolved star that has burned its nuclear fuel.

There is one known binary star pair consisting of two white dwarfs that is also a "runaway" star, moving very rapidly through the galaxy. 

A new study concludes that this runaway star was probably ejected from a dense stellar cluster. 

Credit: NASA/Hubble

CfA astronomers made a remarkable and fortuitous discovery in 2005: an extremely fast moving star, clocked going over three million kilometers an hour.

It appears to have been ejected from the vicinity of the galactic center's supermassive black hole around 80 million years ago by powerful gravitational effects as it swung past the black hole.

Racing outward from the galaxy, the star lends added credibility to the picture of a massive black hole at the galactic center, and to calculations of how black holes might interact with their stellar environments.

Other hypervelocity stars and less fast-moving runaway stars have also been found. Most of them have been accelerated by one of the two other gravitational mechanisms: ejection from a dense cluster of stars as random motions bring it into a slingshot-like orbit, or ejection from a supernova binary system after the supernovae explodes and frees it from its orbit.

A binary star is a pair of stars that orbit each other, and many (perhaps most) stars are members of binary systems.

So far, there have been no hypervelocity binary stars discovered. They have been predicted, however, with at least one theory proposing that the discovery of a hypervelocity binary pair might indicate that the nuclear black hole is itself a binary pair.

More information: Kilic, M. et al. The Runaway Binary LP 400/22 is Leaving the Galaxy, MNRAS, 434, 3582, 2013.

GRB Explosion illuminates invisible galaxy in the dark ages

Before light from the gamma-ray burst arrives at the Earth for astronomers to study, it passes through interstellar gas in its host galaxy (close-up view, left), and intergalactic gas between the distant galaxy and us (wide view, right).

This gas filters the light by absorbing some colors and leaves a signature on the light that can be seen in its spectrum.

This "signature" allows scientists to characterize the gamma-ray burst, its environment, and the material between us and the distant galaxy.

Credit: Gemini Observatory/AURA, artwork by Lynette Cook

More than 12 billion years ago a star exploded, ripping itself apart and blasting its remains outward in twin jets at nearly the speed of light. At its death it glowed so brightly that it outshone its entire galaxy by a million times.

This brilliant flash traveled across space for 12.7 billion years to a planet that hadn't even existed at the time of the explosion - our Earth.

By analyzing this light, astronomers learned about a galaxy that was otherwise too small, faint and far away for even the Hubble Space Telescope to see.

Ryan Chornock

"This star lived at a very interesting time, the so-called dark ages just a billion years after the Big Bang," says lead author Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA).

"In a sense, we're forensic scientists investigating the death of a star and the life of a galaxy in the earliest phases of cosmic time," he adds.

The star announced its death with a flash of gamma rays, an event known as a gamma-ray burst (GRB).

GRB 130606A was classified as a long GRB since the burst lasted for more than four minutes.

It was detected by NASA's Swift spacecraft on June 6th. Chornock and his team quickly organized follow-up observations by the MMT Telescope in Arizona and the Gemini North telescope in Hawaii.

"We were able to get right on target in a matter of hours," Chornock says. "That speed was crucial in detecting and studying the afterglow."

A GRB afterglow occurs when jets from the burst slam into surrounding gas, sweeping that material up like a snowplow, heating it, and causing it to glow.

As the afterglow's light travels through the dead star's host galaxy, it passes through clouds of interstellar gas.

Chemical elements within those clouds absorb light at certain wavelengths, leaving "fingerprints." By splitting the light into a rainbow spectrum, astronomers can study those fingerprints and learn what gases the distant galaxy contained.

All chemical elements heavier than hydrogen, helium, and lithium had to be created by stars.

As a result those heavy elements, which astronomers collectively call "metals," took time to accumulate. Life could not have existed in the early universe because the elements of life, including carbon and oxygen, did not exist.

Chornock and his colleagues found that the GRB galaxy contained only about one-tenth of the metals in our solar system. Theory suggests that although rocky planets might have been able to form, life probably could not thrive yet.

"At the time this star died, the universe was still getting ready for life. It didn't have life yet, but was building the required elements," says Chornock.

At a redshift of 5.9, or a distance of 12.7 billion light-years, GRB 130606A is one of the most distant gamma-ray bursts ever found.

"In the future we will be able to find and exploit even more distant GRBs with the planned Giant Magellan Telescope," says Edo Berger of the CfA, a co-author on the publication.

Mini-Neptunes: First transiting planets in a star cluster discovered

In the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars. Credit: Michael Bachofner

All stars begin their lives in groups. Most stars, including our Sun, are born in small, benign groups that quickly fall apart.

Others form in huge, dense swarms that survive for billions of years as stellar clusters.

Within such rich and dense clusters, stars jostle for room with thousands of neighbors while strong radiation and harsh stellar winds scour interstellar space, stripping planet-forming materials from nearby stars.

It would thus seem an unlikely place to find alien worlds.

Yet 3,000 light-years from Earth, in the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars.

The discovery, published in the journal Nature, shows that planets can develop even in crowded clusters jam-packed with stars.

"Old clusters represent a stellar environment much different than the birthplace of the Sun and other planet-hosting field stars," says lead author Soren Meibom of the Harvard-Smithsonian Center for Astrophysics (CfA).

"And we thought maybe planets couldn't easily form and survive in the stressful environments of dense clusters, in part because for a long time we couldn't find them."

The two new alien worlds appeared in data from NASA's Kepler spacecraft. Kepler hunts for planets that transit, or cross in front of, their host stars.

During a transit, the star dims by an amount that depends on the size of the planet, allowing the size to be determined.

Kepler-66b and Kepler-67b are both less than three times the size of Earth, or about three-fourths the size of Neptune (mini-Neptunes).

Of the more than 850 known planets beyond our solar system, only four - all similar to or greater than Jupiter in mass - were found in clusters.

Kepler-66b and -67b are the smallest planets to be found in a star cluster, and the first cluster planets seen to transit their host stars, which enables the measurement of their sizes.

Meibom and his colleagues have measured the age of NGC 6811 to be one billion years. Kepler-66b and Kepler-67b therefore join a small group of planets with precisely determined ages, distances, and sizes.

Considering the number of stars observed by Kepler in NGC 6811, the detection of two such planets implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars (stars not within a cluster or association) in the Milky Way galaxy.

"These planets are cosmic extremophiles," says Meibom. "Finding them shows that small planets can form and survive for at least a billion years, even in a chaotic and hostile environment."

More information: Paper: DOI: 10.1038/nature12279