Dwarf Planets: Science and Facts

Pluto was demoted to dwarf planet status in 2006, joining Eris, Haumea, Makemake and Ceres.

Credit: Karl Tate, SPACE.com

In 2006, an assembly of the International Astronomical Union voted to define a planet as a celestial body that a) is in orbit around the sun, 2) has enough mass for its gravity to pull it into a rounded shape (hydrostatic equilibrium), and 3) has cleared the neighborhood of its orbit of other, smaller objects.

This last criterion is the point at which planets and dwarf planets differ. A planet's gravity either attracts or pushes away the smaller bodies that would otherwise intersect its orbit; the gravity of a dwarf planet is not sufficient to make this happen.

Astronomers estimate that there could be as many as 200 dwarf planets in the solar system and Kuiper Belt. But the differences between planets and dwarf planets may not be obvious at first. 

New Studies Propose Two New Planets Beyond Pluto

New calculations from an international team of researchers suggest that there could be two unknown planets beyond Pluto.

In two new studies, published in the Monthly Notices of the Royal Astronomical Society, scientists from the University of Madrid and the University of Cambridge posit that two, or more, unknown planets are responsible for unusual behaviour beyond Neptune.

Objects, in that far-flung part of the Solar System, don’t act like we believe they should: the paths they move along do not have the orbital inclinations and axes that astronomers would expect to see, based on current theory of what researchers call "trans-Neptunian objects."

Unless, the astrophysicists posit, there’s something else out there, and, after considering the effects of the “Kozai mechanism,” the effect an object orbiting further out from a gravitational source can have one orbiting futher in, they now believe there are at least two mysterious planets at play.

“Our results may be truly revolutionary for astronomy,” says co-author Carlos de la Fuente Marcos in a news release, but the team says this is still a hypothesis.

To prove it, scientists will have to overcome two hurdles. Not only does the new theory challenge astrophysicists' current thinking about how the solar system came to be, but the sample size used in the study’s calculations contains only 13 objects.

However, the team promises that with a larger sample size, coming soon, and new research (a recently-discovered planet-in-process is much further away from a star than scientists suspected to be possible), they’ll soon have even more evidence that our solar system may be even bigger than thought.

More Information
'Flipping minor bodies: what comet 96P/Machholz 1 can tell us about the orbital evolution of extreme trans-Neptunian objects and the production of near-Earth objects on retrograde orbits' published in the Monthly Notices of the Royal Astronomical Society - 10.1093/mnras/stu2230

NASA New Horizon Mission 2015, the year of the Dwarf Planet

This “movie” of Pluto and its largest moon, Charon, by NASA’s New Horizons spacecraft taken in July 2014 clearly shows that the barycenter, the centre of mass of the two bodies, resides outside (between) both bodies. 

The 12 images that make up the movie were taken by the spacecraft’s best telescopic camera, the Long Range Reconnaissance Imager (LORRI), at distances ranging from about 267 million to 262 million miles (429 million to 422 million kilometers). 

Charon is orbiting approximately 11,200 miles (about 18,000 kilometers) above Pluto’s surface. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Southwest Research Institute

The New Horizons mission became the first mission of NASA's New Frontiers program, beginning development in 2001.

The probe was launched on January 19, 2006, atop an Atlas V 551 (5 solid rocket boosters plus a third stage).

Utilising more compact and lightweight electronics than its predecessors to the outer planets, Pioneer 10 & 11, and Voyager 1 & 2, the combination of reduced weight, a powerful launch vehicle, plus a gravity assist from Jupiter has lead to a nine year journey.

On December 6, 2014, New Horizons was taken out of hibernation for the last time and now remains powered on until the Pluto encounter.

The arrival date of New Horizon is July 14, 2015. A telescope called the Long Range Reconnaissance Imager (LORRI) has permitted the commencement of observations while still over 240 million kilometers (150 million miles) from Pluto.

The first stellar-like images were taken while still in the Asteroid belt in 2006.

From Dream to Discovery: Inside NASA Engineering - Video

Experience the challenges of the next generation of space exploration in this brand-new Planetarium show.

By using exciting real-life projects like NASA's James Webb Space Telescope (JWST) and the New Horizons mission to Pluto, the show highlights the extreme nature of spacecraft engineering and the life cycle of a space mission, from design and construction to the rigors of testing, launch, and operations.

Blast off and take the voyage with NASA!

NASA's New Horizons Spacecraft will Encounter Dwarf Planet Pluto in 2015

This artist's rendering shows NASA's New Horizons spacecraft during its flyby of Pluto and its moons on July 14, 2015. 

The spacecraft awoke from its final hibernation period on Dec. 6, 2014 in preparation for the epic Pluto encounter at the edge of the solar system.

Credit: Johns Hopkins University Applied Physics Laboratory /Southwest Research Institute (JHUAPL/SwRI)

Pluto, get ready for your close-up: A NASA spacecraft has roused itself from the final slumber of its nine-year trek to the edge of the solar system, setting the stage for the first close encounter with Pluto next year.

The New Horizons spacecraft, currently located 2.9 billion miles (4.6 billion kilometers) from Earth, had been in hibernation since August, with most of its systems turned off to reduce wear, but late Saturday (Dec. 6), mission scientists received a confirmation signal from New Horizons at the probe's Mission Operations Center here at the Johns Hopkins Applied Physics Laboratory.

The probe is now wide awake for its 2015 flyby of Pluto.

At the time of its wakeup call, New Horizons spacecraft was just over 162 million miles (261 million km) from Pluto.

About 20 people gathered in a conference room here at APL to await the signal from New Horizons.

The Dwarf Planet is officially the Largest in Solar System

This image shows an artist impression of the Dwarf Planet Eris.

Since Eris is larger than the Dwarf Planet Pluto, it is presented as the tenth planet.

However, a long-lasting debate over the status of Pluto forced the International Astronomical Union (IAU) to develop a precise definition of the term planet. 

On August 24, 2006, the IAU adopted a resolution, under which both Pluto and Eris were classified as "dwarf planets" and subsequently added to the Minor Planet Catalogue.

Our universe is full of mysteries but there are a few things we know for certain.

For instance, that the Earth orbits the Sun and not vice versa, or that there are eight planets in the solar system. If you still believe in the latter, you probably have not heard of Eris.

This is an image of the dwarf planet Eris (center) and its companion satellite Dysnomia (at 9 o'clock position) taken with NASA's Hubble Space Telescope on Aug. 30, 2006. Hubble observations were obtained on Dec. 3, 2005 and Aug. 30, 2006 using the Advanced Camera for Surveys.

Credit: Hubblesite

Eris is the largest dwarf planet discovered in 2005 using the Hubble Telescope and was initially described by NASA as the Solar System's tenth planet.

Eris is 27% larger than Pluto, has a diameter of 2.3 kilometers and one companion satellite (moon) called Dysnomia.

The planet orbits the sun at a distance of 96.4 astronomical units, taking 557 years to complete one lap.

Whilst it sounds like a fully-fledged planet, the word 'dwarf' tends to instill confusion. Eris is what astronomers call a plutoid; a trans-Neptunian object located in the part of the solar system known as the Kuiper belt.

A dwarf planet is now officially defined as a "celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravity, but that unlike a planet has not cleared its orbit of other objects."

The number of known planets in the solar system was therefore reduced to eight, as it was before Pluto's discovery in 1930.

With the new status Eris was granted its present name. Previously, the newly discovered space object was informally called Xena after a character from the popular television series Xena: Warrior Princess, but given the discord it caused in the astronomical community, the name of the Greek goddess Eris, a personification of strife, suits this planet like no other.

New Horizons has One Last Sleep til Pluto

On Aug. 25, New Horizons crossed the orbit of Neptune-the last planetary orbit crossing during cruise.

Now the spacecraft is outbound for Pluto.

On Aug. 29 the team put New Horizons into hibernation for the final time, prior to its encounter with Pluto.

This last hibernation lasts 99 days and ends on Dec. 6.

After seven-plus years of hibernating through most of the 2.5-billion mile journey from Jupiter to Pluto and the inner reaches of the Kuiper Belt, the spacecraft has reached the final, short leg of cruise.

New Horizons will be re-awakened for the last time in just 10 weeks. Once this has been done, 'encounter' preparations will begin, and six weeks later, the Pluto encounter itself will begin.

At that time, the New Horizons team will have good cause to celebrate. They will have reached the outer end of our Solar System, twenty-five years after first wondering if Pluto might someday be explored.

This summer's eighth and final "pre-Pluto" spacecraft and payload Active Check Out (ACO-8) lasted from June through late August. All spacecraft subsystems-both prime and backup-were checked out and were found to be operating successfully.

Additionally, the team performed their first course-correction since 2010, uploaded the final autonomy system software for the encounter, checked out all seven payload instruments, conducted some final instrument calibrations, and performed their first optical navigation campaign to home in on Pluto using New Horizons' Long Range Reconnaissance Imager (LORRI).

Those activities went well, and so did many others, including more sampling of the heliospheric plasma and dust environment with PEPSSI, SWAP and Student Dust Counter instruments.

The only real anomaly of the entire ACO-8 was a failed startup of a single Alice ultraviolet spectrometer observation.

That observation was to study the distribution of interplanetary hydrogen near Neptune's orbit; it failed because Alice was much colder than was planned, and onboard software "safed" (or turned off) Alice's high-voltage power supply when it took too long to get to its set point voltages.

The team have now learned that they need to adjust some timing settings for future power-ons when Alice will be as cold or even colder, On its approach to Pluto.

Additionally, ACO-8 was the subject of a recent news story from New Horizons: While testing the methods to be used to search for hazards in the Pluto system on approach, the spacecraft detected Pluto's little moon Hydra.

The team didn't think it would be possible to see Hydra until early in 2015, when the spacecraft was much closer, but science team members John Spencer and Hal Weaver found Hydra in the July hazard-sequence test.

The New Horizons team sees the early detection of Hydra as good news, because it anticipates their ability to detect currently unknown moons and rings close to Pluto.

New Horizons: Awaiting New Results on Pluto's Atmosphere

Artist's impression of Pluto, with its wispy atmosphere.

Data from New Horizons' Alice ultraviolet spectrograph will answer a full range of questions about the composition and structure of that atmosphere.

What is Pluto's atmosphere like? It seems like I've been wondering about that for decades!

We've known so little for so long about Pluto's atmosphere, other than it's low-pressure, made mostly of molecular nitrogen (with a little methane and carbon monoxide mixed in) and may be quite extended, it's nice to realise that we'll know a whole lot more after New Horizons visits in summer 2015.

Alice UV spectrograph

My professional interests on New Horizons lie with Pluto's upper atmosphere, what it's made of, how it interacts with space, and how it is processed by sunlight into different gases and aerosols.

A problem in planning atmospheric observations for New Horizons during the flyby is that we really don't know what to expect.

Only a few models have been made that try to predict the composition of Pluto's atmosphere, and they don't agree very much with each other because of the many present uncertainties.

So our plans generally include a lot of survey-type observations, where we try not to assume too much about what we will detect, but are ready for anything.

The best example of this is the Pluto solar occultation observation.

Just Joking!

The Alice ultraviolet spectrograph will watch the Sun set (and then rise again) as New Horizons flies through Pluto's shadow, about an hour after closest approach.

Watching how the different colours of sunlight fade (and then return) as New Horizons enters (and leaves) the shadow will tell us nearly all we could ask for about composition (all gases have unique absorption signatures at the ultraviolet wavelengths covered by Alice) and structure (how those the absorption features vary with altitude will tell us about temperatures, escape rates and possibly about dynamics and clouds).

When the New Horizons data start coming down, these are the data I'll be waiting for the most!

New Horizons: Team spot Charon, the tiny moon of Pluto

Artist’s conception of the New Horizons spacecraft flying past Pluto and Charon, one of the dwarf planet’s moons. 

Credit: Johns Hopkins University/APL 

The New Horizons team spotted Charon, the tiny moon of Pluto in July, about six months ahead of when they expected to.

You can check it out in the images below.

The find is exciting in itself, but it also bodes well for the spacecraft's search for orbital debris to prepare for its close encounter with the system in July 2015.

Most of Pluto's moons were discovered while New Horizons was under development, or already on its way.

Mission planners are thus concerned that there could be moons out there that aren't discovered yet, moons that could pose a danger to the spacecraft if it ended up in the wrong spot at the wrong time.

That's why the team is engaging in long-range views to see what else is lurking in Pluto's vicinity.

"We're thrilled to see it, because it shows that our satellite-search techniques work, and that our camera is operating superbly, but it's also exciting just to see a third member of the Pluto system come into view, as proof that we're almost there," stated science team member John Spencer, of the Southwest Research Institute.

Hydra was spotted using the spacecraft's Long Range Reconnaissance Imager (LORRI), which took 48 images of 10 seconds apiece between July 18 and July 20.

Then the team used half the images, the ones that show Hydra better, to create the images you see above.

The spacecraft was still 267 million miles (430 million kilometers) from Pluto when the images were taken.

Another moon discovered around the same time as Hydra, Nix, is still too close to be seen given it's so close to Pluto, but just wait.

Meanwhile, scientists are busily trying to figure out where to send New Horizons after Pluto.

In July, researchers using the Hubble Space Telescope began a full-scale search for a suitable Kuiper Belt Object, which would be one of trillions of icy or rocky objects beyond Neptune's orbit.

Flying past a KBO would provide more clues as to how the Solar System formed, since these objects are considered leftovers of the chunks of matter that came together to form the planets.

Watch the difference: Pluto’s moon Hydra stands out in these images taken by the New Horizons spacecraft on July 18 and 20, 2014. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Southwest Research Institute

NASA Probe Sees Pluto and Its Moon Charon - Video

A NASA spacecraft hurtling toward a 2015 rendezvous with Pluto has captured an amazing video of the dwarf planet and its largest moon, Charon.

The New Horizons probe used its onboard telescope to record a video of Charon and Pluto locked in their orbital dance.

The movie is composed of 12 individual images taken from July 19 through July 24, almost exactly one year before the spacecraft's flyby of the distant dwarf planet, which is scheduled for July 14, 2015.

"The image sequence showing Charon revolving around Pluto set a record for close-range imaging of Pluto, they were taken from 10 times closer to the planet than the Earth is," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said in a statement. "But we'll smash that record again and again, starting in January, as approach operations begin."

"We are really excited to see our target and its biggest satellite in motion from our own perch, less than a year from the historic encounter ahead!" he added.

New Horizons was about 265 million miles (426 million kilometers) from Pluto when it took the 12 photos, researchers said.

New Horizons baseline spacecraft design. 

Image Credit: The Boeing Company

ALMA pinpoints Pluto to help guide NASA's New Horizons spacecraft

The cold surface of Pluto and its largest moon Charon as seen with ALMA on July 15, 2014. 

Credit: NRAO/AUI/NSF

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) are making high-precision measurements of Pluto's location and orbit around the Sun to help NASA's New Horizons spacecraft accurately home in on its target when it nears Pluto and its five known moons in July 2015.

Though observed for decades with ever-larger optical telescopes on Earth and in space, astronomers are still working out Pluto's exact position and path around our Solar System.

ALMA - The Atacama Array

This lingering uncertainty is due to Pluto's extreme distance from the Sun (approximately 40 times farther out than the Earth) and the fact that we have been studying it for only about one-third of its orbit.

Pluto was discovered in 1930 and takes 248 years to complete one revolution around the Sun.

"With these limited observational data, our knowledge of Pluto's position could be wrong by several thousand kilometers, which compromises our ability to calculate efficient targeting maneuvers for the New Horizons spacecraft," said New Horizons Project Scientist Hal Weaver, from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

NASA's New Horizons spacecraft

The New Horizons team made use of the ALMA positioning data, together with newly analyzed visible light measurements stretching back to Pluto's discovery, to determine how to perform the first such scheduled course correction for targeting, known as a Trajectory Correction Maneuver (TCM), in July.

This maneuver helped ensure that New Horizons uses the minimum fuel to reach Pluto, saving as much as possible for a potential extended mission to explore Kuiper Belt objects after the Pluto system flyby is complete.

Animated image of ALMA data showing the motion of the moon Charon around the icy dwarf planet Pluto. 

Credit: B. Saxton (NRAO/AUI/NSF)

To prepare for this first TCM, astronomers needed to pinpoint Pluto's position using the most distant and most stable reference points possible.

Finding such a reference point to accurately calculate trajectories of such small objects at such vast distances is incredibly challenging.

Normally, stars at great distances are used by optical telescopes for astrometry (the positioning of things on the sky) since they change position only slightly over many years.

For New Horizons, however, even more precise measurements were necessary to ensure its encounter with Pluto would be as on-target as possible.

The most distant and most apparently stable objects in the Universe are quasars, galaxies more than 10 billion light-years away.

Though quasars appear very dim to optical telescopes, they are incredibly bright at radio wavelengths, particularly the millimeter wavelengths that ALMA can see.

Ed Fomalont

"The ALMA astrometry used a bright quasar named J1911-2006 with the goal to cut in half the uncertainty of Pluto's position," said Ed Fomalont, an astronomer with the National Radio Astronomy Observatory in Charlottesville, Virginia, and currently assigned to ALMA's Operations Support Facility in Chile.

ALMA was able to study Pluto and its largest moon Charon by picking up the radio emission from their cold surfaces, which are about 43 degrees Kelvin (-230 degrees Celsius).

The team first observed these two icy worlds in November 2013, and then three more times in 2014, once in April and twice in July. Additional observations are scheduled for October 2014.

"By taking multiple observations at different dates, we allow Earth to move along its orbit, offering different vantage points in relation to the Sun," said Fomalont.

"Astronomers can then better determine Pluto's distance and orbit." This astronomical technique is called measuring Pluto's parallax.

"We are very excited about the state-of-the-art capabilities that ALMA brings to bear to help us better target our historic exploration of the Pluto system," said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado.

"We thank the entire ALMA team for their support and for the beautiful data they are gathering for New Horizons."

What is beneath the cracked surface of Pluto's moon Charon?

An artist’s concept of Pluto as viewed from the surface of one its moons. 

Pluto is the large disk at the center of the image. Charon is the smaller disk to the right. 

Credit: NASA, ESA and G. Bacon (STScI)

Is there evidence of an ocean-past or present-waiting to surprise us on Charon?

It isn't impossible. In fact, it might be likely.

What used to be the smallest planet in our solar system has, comparatively, the biggest moon.

Pluto, now classified as a dwarf planet, has a moon, Charon, almost 1/8th its own mass and almost half its physical volume.

Our Moon, by comparison, has about 1% of the Earth's mass and only 2% of its volume.

Charon is so large compared to Pluto that some astronomer's consider the two to be a sort of binary dwarf-planet system, as opposed to a moon-and-planet system.

Both Charon and our Moon are believed to have formed in the same way: when they were knocked off their parent planets.

Enormous collisions liquified parts of the Earth and Pluto. The debris was thrown into orbit where it later cooled.

In the process of cooling into solid bodies around the Earth and Pluto, the Moon and Charon became locked to their parent planets' orbits.

That locking of the planets to moons results in tides: here on Earth, on the Moon, and, we believe, on Pluto and Charon.

An analysis by scientists at Goddard suggests that tides on Pluto and Charon could have been especially high as Charon cooled.

This is because the part of Pluto knocked into orbit didn't get very far. Charon formed incredibly close to Pluto: only 19,000 km (12,000 miles) away.

By comparison, our Moon is currently 384,000 km (238,855 mi) from Earth. Initially, the orbit might not have been very circular, either: it might have been more eccentric or elliptical-shaped.

Eccentrically-moving, close-by Charon would have pulled on Pluto, and Pluto would have pulled back, resulting in heating of both planets and, maybe, an ocean under Charon's ice shell.

Alyssa Rhoden

Depending on exactly how Charon's orbit evolved, particularly if it went through a high-eccentricity phase, there may have been enough heat from tidal deformation to maintain liquid water beneath the surface of Charon for some time," said Alyssa Rhoden of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"Using plausible interior structure models that include an ocean, we found it wouldn't have taken much eccentricity (less than 0.01) to generate surface fractures like we are seeing on Europa."

Artist impression of the New Horizons spacecraft as it approached Jupiter en route to Pluto. 

Credit: NASA

On icy moons like Europa and Enceladus, tidal forces exerted by their parent planets cause massive surface cracks to form.

Those cracks are easily appreciated by passing spacecraft. According to Rhoden and colleagues' model, Charon's surface should be similarly cracked.

We expect to see evidence of this fractured surface geology as the New Horizons spacecraft approaches Pluto. New Horizons will pass directly over Pluto and Charon, briefly, on July 15th 2015.

Charon was discovered thirty-five years ago, in 1978, but well-photographed for the first time by New Horizons in 2013.

With the 2015 close-up just around the corner, scientists are working swiftly to make best use of surface photographs returned by the spacecraft.

New Horizons will give us the ability to resolve objects as small as a football field on part of the surface of Pluto and Charon.

With pictures of that detail and models such as this one, we may be able to look backwards in time to determine details about both bodies, such as how thick their ice shells were when they formed.

Studying patterns of fractures in Charon's surface is critical to building accurate models of the ice shell and layers beneath.

"Our model predicts different fracture patterns on the surface of Charon depending on the thickness of its surface ice, the structure of the moon's interior and how easily it deforms, and how its orbit evolved," said Rhoden.

"By comparing the actual New Horizons observations of Charon to the various predictions, we can see what fits best and discover if Charon could have had a subsurface ocean in its past, driven by high eccentricity."

The oceans of certain icy moons with surface fractures are considered to be places where extraterrestrial life might be found.

Like Charon, Europa and Enceladus are very cold and very distant from the sun. In all three cases, the formation and maintenance of life would depend upon a reliable energy source as well as elements that can participate in the chemistry of life, such as carbon, nitrogen, and phosphorus.

New Horizons Long Range Reconnaissance Imager (LORRI) composite image showing the detection of Pluto’s largest moon, Charon. 

When these images were taken on July 1 and July 3, 2013, the New Horizons spacecraft was still about 550 million miles (880 million kilometers) from Pluto.

On July 14, 2015, the spacecraft is scheduled to pass just 7,750 miles (12,500 kilometers) above Pluto’s surface, where LORRI will be able to spot features about the size of a football field. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Southwest Research Institute (SRI)

It is unknown if a potential ocean on Charon may have harbored these ingredients or if the ocean there existed for long enough for life to form.

The same questions apply to any ancient ocean on any moon in our Solar System or any other. The first step on Charon is to find the fractures, and then go looking for the warmth that liquid water.

"Since it's so easy to get fractures, if we get to Charon and there are none, it puts a very strong constraint on how high the eccentricity could have been and how warm the interior ever could have been," said Rhoden.

"This research gives us a head start on the New Horizons arrival, what should we look for and what can we learn from it. We're going to Pluto and Pluto is fascinating, but Charon is also going to be fascinating."

Hubble Begins Search Beyond Pluto For Potential Flyby Targets

This is an artist's rendering of the New Horizons spacecraft encountering a Kuiper Belt object - a city-sized icy relic left over from the birth of our solar system. 

The sun, more than 4.1 billion miles (6.7 billion kilometers) away, shines as a bright star embedded in the glow of the zodiacal dust cloud. 

Jupiter and Neptune are visible as orange and blue "stars" to the right of the sun. 

Image courtesy JHUAPL /SwRI.

After careful consideration and analysis, the Hubble Space Telescope Time Allocation Committee has recommended using Hubble to search for an object the Pluto-bound NASA New Horizons mission could visit after its flyby of Pluto in July 2015.

The planned search will involve targeting a small area of sky in search of a Kuiper Belt object (KBO) for the outbound spacecraft to visit.

The Kuiper Belt is a vast debris field of icy bodies left over from the solar system's formation 4.6 billion years ago.

A KBO has never been seen up close because the belt is so far from the sun, stretching out to a distance of 5 billion miles into a never-before-visited frontier of the solar system.

"I am pleased that our science peer-review process arrived at a consensus as to how to effectively use Hubble's unique capabilities to support the science goals of the New Horizons mission," said Matt Mountain, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

Fully carrying out the KBO search is contingent on the results from a pilot observation using Hubble data.

The space telescope will scan an area of sky in the direction of the constellation Sagittarius to try and identify any objects orbiting within the Kuiper Belt.

To discriminate between a foreground KBO and the clutter of background stars in Sagittarius, the telescope will turn at the predicted rate that KBOs are moving against the background stars.

In the resulting images, the stars will be streaked, but any KBOs should appear as pinpoint objects.

If the test observation identifies at least two KBOs of a specified brightness it will demonstrate statistically that Hubble has a chance of finding an appropriate KBO for New Horizons to visit.

At that point, an additional allotment of observing time will continue the search across a field of view roughly the angular size of the full moon.

Astronomers around the world apply for observing time on the Hubble Space Telescope. Competition for time on the telescope is extremely intense and the requested observing time significantly exceeds the observing time available in a given year.

Proposals must address significant astronomical questions that can only be addressed with Hubble's unique capabilities, and are beyond the capabilities of ground-based telescopes.

The proposals are peer reviewed annually by an expert committee, which looks for the best possible science that can be conducted by Hubble and recommends to the Space Telescope Science Institute director a balanced program of small, medium, and large investigations.

Though Hubble is powerful enough to see galaxies near the horizon of the universe, finding a KBO is a challenging needle-in-haystack search.

A typical KBO along the New Horizons trajectory may be no larger than Manhattan Island and as black as charcoal.

Herschel’s population of trans-Neptunian objects

Herschel’s population of trans-Neptunian objects.

Credit: ESA

ESA’s Herschel space observatory has observed 132 of the known 1400 cold worlds that inhabit a region of the Solar System beyond the orbit of Neptune, some 4.5–7.5 billion km from the Sun.

These ‘trans-Neptunian objects’ (TNOs), include worlds such as Pluto, Eris, Haumea and Makemake, and make up a vast population of such objects thought to occupy these far-flung reaches of the Solar System.

TNOs are particularly cold, at around –230ºC, but these low temperatures lend themselves to observations by Herschel, which observes at far-infrared to sub-millimetre wavelengths.

Indeed, the space observatory observed the thermal emission from 132 such objects during its nearly four-year lifetime.

These measurements provided their sizes and albedos (the fraction of visible light reflected from the surface), properties that are not otherwise easily accessible.

The graphic presented here shows a sample of the population of TNOs observed with Herschel, arranged to showcase these properties.

What is most striking is their diversity. They range from just below 50 km to almost 2400 km in diameter; Pluto and Eris are the largest.

Two worlds have distinctly elongated shapes: Haumea (seen in white) and Varuna (brown). Some even host their own moons (not shown).

The albedo measurement implies a variety of surface compositions: low albedo (brown) is an indication of dark surface materials, such as organic material, while higher albedo (white) suggests pure ices.

TNOs are thought to be some of the most primitive remnants of the planet-forming era. Thus the results of the Herschel “TNOs are cool: A survey of the trans-Neptunian region” open key time programme are being used to test different models of Solar System formation and evolution.

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

Pluto: Three possible models of Dwarf Planet ahead of New Horizons visit

Interior structure models assumed for Pluto.

Two space researchers, Amy Barr, with Brown University and Geoffrey Collins with Wheaton College, have published a paper in the journal Icarus in which they describe three possible interior models of the former planet Pluto.

They suggest the possibilities include: 

1. an undifferentiated rock/ice mixture, 
2. a differentiated rock/ice mixture, and an ocean covered with ice. 
3. The third possibility suggests the likelihood, they claim, of tectonic action on the dwarf planet.

Pluto

A close up view of the planet by space probe New Horizons due to arrive next year, should help clarify which scenario is most likely.

Amy Barr

Scientists believe that Pluto came to exist as it does today, in part due to a collision billions of years ago that led also to the formation of its moon Charon.

Charon

When celestial bodies collide, not only do they knock each other around, they produce heat—heat, the researchers suggest that could still be evident today.

Barr and Collins are leading towards a theory that suggests that shortly after impact, Pluto and Charon were much closer together, the gravity attraction between them would have caused both to be egg shaped.

As time passed, melted ice from the impact would have created an icy crust on top of an ocean on Pluto, and then, as Charon moved farther away, the attractive pull would have diminished, causing ice plates to form and crack against one another, a form of tectonics.

Geoffrey Collins

If that were the case, the two add, then in all likelihood, when New Horizons begins sending back images, they should see evidence of such tectonic action—plate edges thrust into the air, for example.

There's just one catch, Pluto circles the sun in an elliptical orbit, thus sometimes it's much closer to the sun than other times.

When near, it has a defined atmosphere, when far away however, its atmosphere actually freezes to its surface, something that could hide ridges in the ice and thus evidence of both tectonic activity and an ocean beneath the crust of ice.

New Horizons

Artist concept of New Horizons spacecraft.

Johns Hopkins University Applied Physics Laboratory (JHUAPL) /Southwest Research Institute (SwRI)

Since New Horizons will arrive during a time when its atmosphere is frozen to the surface, it might be difficult to determine which of the three proposed models actually describes the relationship between its exterior and interior.

Barr and Collins are optimistic that even in such a scenario, ridges should be apparent, proving that beneath Pluto's icy surface, lies an ocean, one that future researchers might one day sample.

More information: Tectonic Activity on Pluto After the Charon-Forming Impact, Icarus, Available online 4 April 2014. dx.doi.org/10.1016/j.icarus.2014.03.042 . Available on Arxiv: xxx.lanl.gov/abs/1403.6377

NASA WISE: Uranus and the Planet X myth - debunked

The hunt for Planet X began after Uranus (pictured) was first discovered in 1781 with astrologers hoping it could explain the wobbly orbit of Uranus around the sun

It was an elusive planet that for 200 years appeared to explain Uranus's wobbly orbit and there was the sister sun theorised to be near our solar system that caused asteroids to swerve toward Earth.

There is just one problem: neither "Planet X" nor "Nemesis" ever existed, researchers now say. Although, there is still a trace of doubt.

"The outer solar system probably does not contain a large gas giant planet ("Planet X"), or a small, companion star ("Nemesis")," concluded University of Pennsylvania astronomer Kevin Luhman, who directed the study using NASA's Wide-field Infrared Survey Explorer (WISE) telescope.

The results were published in the most recent edition of The Astrophysical Journal.

Most theories had estimated Planet X to be up to four times the size of Jupiter—the biggest planet in our solar system.

They suggested it would be found some 1,486 billion kilometers (923 billion miles) from the sun, or about 10,000 times farther than the Earth's orbit.

But the images gathered by the telescope did not detect any object larger than Jupiter.

Luhman doesn't rule out the possibility that a planet is lurking somewhere in the asteroid belt.

It would be hard to find if it were closely aligned with a bright star that blinds the telescope or were much smaller than had been theorized.

A computer generated NASA montage obtained 29 August 2002 from images collected by the Voyager 2 spacecraft shows Neptune (Lower-L) as it would appear from a spacecraft approaching Triton, Neptune's largest moon

But after this latest survey, Luhman said the odds of finding one are very unlikely: "That is like a one in a hundred chance."

History of Planet X
Scientists first imagined the existence of Planet X in 1781, when they discovered Uranus, a gas giant that astonished astronomers with its orbital variations, apparently incompatible with Newton's laws of gravity.

Observers concluded that these irregularities could be explained by the existence of another, unknown planet that was exerting its own gravitational force.

Attempts to track this mysterious Planet X led to the discovery of Neptune in 1846. But the estimated mass of Neptune couldn't explain the deviations of Uranus's orbit.

That led astronomers to continue their search for Planet X—which, in turn, led to the discovery of Pluto in 1930. But the dwarf planet was also too small to explain Uranus's irregular path around the sun.

Finally, in the 1990s, researchers determined that they had slightly overestimated the mass of Neptune, which meant the planet could in fact be the reason for Uranus's orbital behaviour.

Yet Planet X believers were still not convinced.

Sister sun killed dinosaurs?
The existence of Nemesis, a sun-like star nearby, was first posited in the 1980s. The star, by occasionally coming closer to the sun, interfered with the orbit of comets and asteroids leading them to occasionally hit the Earth.

Collisions like these are blamed for the five mass extinctions over the last 540 million years—the most recent being the dinosaur extinction 65 million years ago.

"So over the years, there have been different pieces of evidence suggesting there might be something there," Luhman explained to reporters but the WISE telescope didn't find anything.

The hunt for Planet X and Nemesis may have turned up empty, but the study did uncover 3,525 stars and brown dwarfs, celestial objects whose mass puts them between a star and a large planet, within 500 light years of the sun.

"Neighbouring star systems that have been hiding in plain sight just jump out in the WISE data," said Ned Wright, a University of California, Los Angeles astronomer who contributed to the study.