Super-Earths May Be Three Times More Common Than Jupiters (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 14 Mar 2006 00:48:20 -0500
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For Release: Embargoed until 8:00 p.m. EST, Monday, March 13, 2006
Release No.: 06-14
Super-Earths May Be Three Times More Common Than Jupiters
Cambridge, MA -- Astronomers have discovered a new "super-Earth" orbiting a red dwarf star located about 9,000 light-years away. This newfound world weighs about 13 times the mass of the Earth and is probably a mixture of rock and ice, with a diameter several times that of Earth. It orbits its star at about the distance of the asteroid belt in our solar system, 250 million miles out. Its distant location chills it to -330 degrees Fahrenheit, suggesting that although this world is similar in structure to the Earth, it is too cold for liquid water or life.
Orbiting almost as far out as Jupiter does in our solar system, this "super-Earth" likely never accumulated enough gas to grow to giant proportions. Instead, the disk of material from which it formed dissipated, starving it of the raw materials it needed to thrive.
"This is a solar system that ran out of gas," says Harvard astronomer Scott Gaudi of the Harvard-Smithsonian Center for Astrophysics (CfA), a member of the MicroFUN collaboration that spotted the planet.
The discovery is being reported today in a paper posted online at
http://arxiv.org/abs/astro-ph/0603276
and submitted to The Astrophysical Journal Letters for publication.
Gaudi performed extensive data analysis that confirmed the existence of the planet. Further analysis simultaneously ruled out the presence of any Jupiter-sized world in the distant solar system.
"This icy super-Earth dominates the region around its star that, in our solar system, is populated by the gas giant planets," said first author Andrew Gould (Ohio State University), who leads MicroFUN.
The team also calculates that about one-third of all main sequence stars may have similar icy super-Earths. Theory predicts that smaller planets should be easier to form than larger ones around low-mass stars. Since most Milky Way stars are red dwarfs, solar systems dominated by super-Earths may be more common in the Galaxy than those with giant Jupiters.
This discovery sheds new light on the process of solar system formation. Material orbiting a low-mass star accumulates into planets gradually, leaving more time for the gas in the protoplanetary disk to dissipate before large planets have formed. Low-mass stars also tend to have less massive disks, offering fewer raw materials for planet formation.
"Our discovery suggests that different types of solar systems form around different types of stars," explains Gaudi. "Sun-like stars form Jupiters, while red dwarf stars only form super-Earths. Larger A-type stars may even form brown dwarfs in their disks."
Astronomers found the planet using a technique called microlensing, an Einsteinian effect in which the gravity of a foreground star magnifies the light of a more distant star. If the foreground star possesses a planet, the planet's gravity can distort the light further, thereby signaling its presence. The precise alignment required for the effect means that each microlensing event lasts for only a brief time. Astronomers must monitor many stars closely to detect such events.
Microlensing is sensitive to less massive planets than the more common planet-finding methods of radial velocity and transit searches.
"Microlensing is the only way to detect Earth-mass planets from the ground with current technology," says Gaudi. "If there had been an Earth-mass planet in the same region as this super-Earth, and if the alignment had been just right, we could have detected it. By adding one more two-meter telescope to our arsenal, we may be able to find up to a dozen Earth-mass planets every year."
The OGLE (Optical Gravitational Lensing Experiment) collaboration initially discovered the microlensed star in April 2005 while peering in the direction of the galactic center, where both foreground and background stars are widespread. OGLE identifies several hundred microlensing events per year, however only a small fraction of those events yield planets. Gaudi estimates that with one or two additional telescopes located in the southern hemisphere to monitor the galactic center, the planet count could jump drastically.
The discovery was made by 36 astronomers, including members of the MicroFUN, OGLE, and Robonet collaborations. The name of the planet is OGLE-2005-BLG-169Lb. OGLE-2005-BLG-169 refers to the 169th microlensing event discovered by the OGLE Collaboration toward the Galactic bulge in 2005, and "Lb" refers to a planetary mass companion to the lens star.
Crucial roles in the discovery were played by OGLE team leader Andrzej Udalski of Warsaw University Observatory and graduate students Deokkeun An of Ohio State and Ai-ying Zhou of Missouri State University. Udalski noticed that this microlensing event was reaching a very high magnification on May 1, and he quickly alerted the MicroFUN group to this fact, since high magnification events are known to be very favorable for planet detection. MicroFUN's regular telescopes were unable to get many images, so MicroFUN leader Gould called the MDM Observatory in Arizona where An and Zhou were observing. Gould asked An and Zhou to obtain a few measurements of the star's brightness over the course of the night, but instead An and Zhou made more than 1000 measurements. This large number of MDM measurements was crucial for the determination the observed signal must really be due to a planet.
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.
Note to editors:
High-resolution artwork to accompany this release is available online at
http://www.cfa.harvard.edu/press/pr0614image.html
*****
Research Communications
Ohio State University
Contact:
Andrew Gould, (614) 292-1892
Written by:
Pam Frost Gorder, (614) 292-9475
3/13/2006
New planet found: Icy 'super-Earth' dominates distant solar system
COLUMBUS , Ohio -- An international collaboration of astronomers has discovered a "super-Earth" orbiting in the cold outer regions of a distant solar system about 9,000 light-years away. The planet weighs 13 times as much as Earth, and at -330 degrees Fahrenheit, it's one of the coldest planets ever discovered outside our solar system.
Andrew Gould, leader of the MicroFUN collaboration and professor of astronomy at Ohio State University, pointed to two key implications of the discovery. "First," Gould said, "this icy super-Earth dominates the region around its star that in our solar system is populated by the gas-giant planets, Jupiter and Saturn. We've never seen a system like this before, because we've never had the means to find them."
"And second," he added, "these icy super-Earths are pretty common. Roughly 35 percent of all stars have them."
The astronomers have submitted a paper on the planet to Astrophysical Journal Letters, and posted a copy on the Internet preprint server arXiv.org.
MicroFUN searches for planets using a phenomenon called gravitational microlensing, which occurs when a massive object such as a star crosses in front of another star shining in the background. The object's strong gravity bends the light rays from the more distant star and magnifies them like a lens. Here on Earth, we see the magnified star get brighter as the lens star crosses in front of it, and then fade as the lens gets farther away.
The OGLE (Optical Gravitational Lensing Experiment) collaboration initially discovered the microlensed star in April 2005. Piecing together their observations, Gould and OGLE leader Andrzej Udalski of Warsaw University Observatory suddenly realized on May 1 that the star was brightening extremely quickly, meaning that it would be exceptionally fertile ground for planet hunting. "It was 4:00 a.m.," Gould said. "I was very excited and frantic to get someone to observe that star."
So Gould called the MDM Observatory in Arizona, where the astronomer on duty happened to be Ohio State graduate student Deokkeun An.
Gould asked An to spare a few minutes during his night's work to occasionally measure the star's brightness. But when An and his co-observer Ai-ying Zhou of Missouri State University heard how intense the signal was, they decided to put aside their own project to take more than 1,000 measurements of the event.
"It's a good thing that they did -- their observations turned out to be critical to our determination that there was a planet," Gould said.
"I thought this was a good chance to take many images of the event, to erase any doubt as to whether this was a planet signal," An remembered. "Since the target could only be seen through the telescope during a short time window, we did not hesitate to follow it."
The event was also observed by astronomers in New Zealand and Hawaii.
But after the astronomers gathered all the data, they faced more difficulties. There remained a chance that the tiny warping they saw in the signal wasn't caused by a planet, and Ohio State graduate student Subo Dong had to write special software to speed their computer models to weed out the other possibilities.
Finally, the models confirmed the presence of a Neptune-mass planet, 13 times heavier than Earth, orbiting a star about half as big as our sun.
Gould suspects that the planet is a bare, icy terrestrial one -- a cold super-Earth. Judging from the absence of Jupiter-like planets in its vicinity, that solar system may lack the gas necessary to make gas planets, he said.
"We can't really tell for sure," he admitted. "If we start getting more statistics on this type of planet, we could piece together a better story."
Until a decade ago, scientists had no evidence of what other solar systems were like. Since then, some 170 planets have been discovered, and most of them have been gas giants similar to Jupiter.
Only a handful of Neptune-mass planets have ever been detected, and only two in the cold outer regions of their solar systems. "The next step is to push the sensitivity of our detection methods down to reach Earth-mass planets," Gould said, "and microlensing is the best way to get there."
The technique is the only one sensitive enough to detect these types of planets, he added. To increase the chance of finding planets like Earth, he would like to see a new generation of telescopes dedicated to microlensing planet searches.
Gould's work was supported by the National Science Foundation and NASA.
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