Astronomers Find a New Type of Planet: The "Mega-Earth" 06-05

Astronomers Find a New Type of Planet: The "Mega-Earth"

Cambridge, MA -

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.

The early universe contained only hydrogen and helium. Heavier elements needed to make rocky planets, like silicon and iron, had to be created in the first generations of stars. When those stars exploded, they scattered these crucial ingredients through space, which then could be incorporated into later generations of stars and planets.

This process should have taken billions of years. However, Kepler-10c shows that the universe was able to form such huge rocks even during the time when heavy elements were scarce.

"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.

This research implies that astronomers shouldn't rule out old stars when they search for Earth-like planets. And if old stars can host rocky Earths too, then we have a better chance of locating potentially habitable worlds in our cosmic neighborhood.

The HARPS-N project is led by the Astronomical Observatory of the Geneva University (Switzerland). The National Institute for Astrophysics (INAF, Italy) has agreed to provide 80 observing nights per year over five years to use HARPS-N coupled to the TNG. The U.S. partners are the Harvard-Smithsonian Center for Astrophysics and the Harvard University Origins of Life Initiative; and the UK partners are the Universities of St. Andrews and Edinburgh, and the Queens University of Belfast.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

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Snow Falling around Infant Solar System 07-27

Snow Falling around Infant Solar System

Cambridge, MA - The sight of a snowfall can thrill children, but the first-ever snow line seen around a distant star gives astronomers an even greater thrill because of what it reveals about the formation of planets and our solar system's history.

Astronomers using the new Atacama Large Millimeter/submillimeter Array (ALMA) telescope have taken the first-ever image of a snow line in an infant solar system. This frosty landmark is thought to play in an essential role in the formation and chemical make-up of planets around a young star.

On Earth, snow lines typically form at high elevations where falling temperatures turn atmospheric moisture to snow. In much the same way, snow lines are thought to form around young stars in the distant, colder reaches of the disks from which solar systems form. Depending on the distance from the star, however, other more exotic molecules can freeze and turn to snow.

Familiar water ice freezes out first, then moving outward in concentric circles other abundant gases like carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) freeze, forming a frost on dust grains, which are the building blocks of planets and comets.

ALMA spotted a never-before-seen CO snow line around TW Hydrae, a young star 175 light-years away from Earth. Astronomers believe this nascent solar system has many of the same characteristics that our own Solar System had when it was just a few million years old. The results were published in Science Express.

"ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of our own Solar System," said Chunhua "Charlie" Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who led the international research team with Karin Oberg, a researcher with Harvard University and the University of Virginia in Charlottesville.

"We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old," said Qi.

Snow lines have, until now, only been detected by their spectral signatures; they have never been imaged directly, so their precise location and extent could not be determined.

This is because snow lines form exclusively in the relatively narrow central plane of a protoplanetary disk. Above and below this region, stellar radiation keeps the gases warm, preventing them from forming ice. Only with the insulating effect of the concentrated dust and gas in the central plane of the disk can temperatures drop sufficiently for CO and other gases to cool and freeze.

Normally, this outer cocoon of hot gas would prevent astronomers from peering inside the disk where the gas had frozen out. "It would be like trying to find a small, sunny patch hidden within a dense fogbank," said Oberg.

The astronomers were able to pierce the intervening CO fog by instead hunting for a different molecule known as diazenylium (N2H+). This fragile molecule is easily destroyed in the presence of CO gas, so would only appear in detectable amounts in regions where CO had frozen out, and is hence a proxy for CO ice.

Diazenylium shines brightly in the millimeter portion of the spectrum, which can be detected by radio telescope like ALMA here on Earth.

ALMA's unique sensitivity and resolution allowed the astronomers to trace the presence and distribution of diazenylium, finding a clearly defined boundary approximately 30 astronomical units (AU) from TW Hydrae (one AU is the Sun-Earth distance).

"Using this technique, we were able to create, in effect, a photonegative of the CO snow in the disk surrounding TW Hydrae," said Oberg. "With this we could see the CO snow line precisely where theory predicts it should be -- the inner rim of the diazenylium ring."

Snow lines, astronomers believe, serve a vital role in the formation of a solar system. They help dust grains overcome their normal tendency to collide and self-destruct by giving the grains a stickier outer coating. 

They also increase the amount of solids available and may dramatically speed up the planet formation process. Since there are multiple snow lines, each may be linked to the formation of specific kinds of planets.

Around a Sun-like star, the water snow line would correspond approximately to the orbit of Jupiter and the CO snow line would roughly correspond to the orbit of Neptune. The transition to CO ice could also mark the starting point where smaller icy bodies like comets and dwarf planets like Pluto would form.

Oberg also points out that the CO snow line is particularly interesting since CO ice is needed to form methanol, which is a building block of more complex organic molecules that are essential for life. Comets and asteroids could then ferry these molecules to newly forming Earth-like planets, seeding them with the ingredients for life.

These observations were made with only a portion of ALMA's eventual full complement of 66 antennas. The researchers hope future observations with the full array will reveal other snow lines and provide additional insights into the formation and evolution of planets.

This release is being issued jointly with NRAO.

ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc..

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

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