"Super-Earth" Discovered Orbiting Distant Star

Stefan Lovgren
for National Geographic News

March 13, 2006

Scientists have discovered a new planet they call a "super-Earth" in a solar system 9,000 light-years away.

The icy, rocky planet, which weighs 13 times as much as Earth, orbits the outer region of its solar system, around a so-called red dwarf star that is about half as big as our sun.

The planet dominates a region similar to the one in our solar system that is populated by the gas giants Jupiter and Saturn. Scientists believe the planet likely didn't accumulate enough gas to grow to giant proportions. (See an interactive map of our solar system.)

"We've never been able to see these failed Jupiter cores before," said Andrew Gould, an astronomer at Ohio State University in Columbus who is leading the research.

He suggests, however, that icy super-Earths are common and that about 35 percent of all stars have them.

Gould leads an international effort called the Microlensing Follow-Up Network (MicroFUN), in which astronomers search for planets using a phenomenon called gravitational microlensing. This occurs when a big object such as a star crosses in front of another star. The gravity of the foreground star bends the light rays from the more distant star and magnifies them like a lens.

"This [technique] has opened up a different area of study observationally for extrasolar planets," Gould said.

If the foreground star has a planet in its orbit, the planet's gravity can distort the light further, and thus signal its presence. The precise alignment required for the effect, however, means that each microlensing event lasts for only a brief time.

"People have thought about this theoretically but just didn't have any way to actually see what was going on," Gould said.

The discovery is reported today in a paper posted at the Web site arXiv.org.

Not Enough Gas

The planet, which has been named OGLE-2005-BLG-169Lb, is probably a mixture of ice and rock. Its terrestrial nature has prompted scientists to dub it a super-Earth, but its distance from its star chills it to -330ºF (-201ºC), making it too cold for liquid water and, presumably, life.

Based on planet formation in our solar system, ice and rock cores that are about ten times as massive as Earth begin to form into gas giants.

However, judging from the absence of Jupiterlike planets in the newly found planet's vicinity, its solar system may have lacked the gas necessary to make gas planets.

"In the disks orbiting low-mass stars, the cores generally do not have enough time to grow to the requisite size to accrete gas before the gas disappears, and the cores are left stranded as Neptune-mass [smaller] planets," said Gregory Laughlin, an astronomy professor at University of California, Santa Cruz.

Scientists believe that smaller planets may form more easily around low-mass stars than larger planets do. Since researchers have found that most stars in our galaxy are red dwarfs (much smaller and dimmer than our sun), solar systems dominated by super-Earths may be more common in our galaxy than those with gas giants.

"[This discovery] is very exciting, because it strongly suggests that Neptune-mass planets are common around low-mass stars, in stark contrast to Jupiter-mass [giant] planets, which appear to be quite rare in orbit around low-mass stars," said Laughlin.

Probing New Frontiers

Until a decade ago scientists didn't know what other solar systems were like. Since then around 170 planets have been discovered. Most of them have been gas giants.

Only a handful of smaller, Neptune-mass planets have been detected, and only two of those were found in the outer regions of their solar systems.

The only technique sensitive enough to detect these types of planets is microlensing. Scientists say the method allows them to probe a new domain of space.

"I think we'll be seeing a lot more detections in the coming years with the microlensing technique," Laughlin said.

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