Mysterious "Super Earth" Is Smallest Known Exoplanet?

Anne Minard
for National Geographic News
April 11, 2008
The eccentric orbit of a Neptune-like planet could be evidence that the distant gas giant has a smaller, rocky neighbor, astronomers say.

If confirmed, the new terrestrial world will be one of the least massive exoplanets known to orbit a sunlike star and only the second known warm "super Earth."

Although they are five or more times as massive as our world, super-Earths are thought to be rocky rather than gaseous—which makes them strong candidates for harboring life.

Ignasi Ribas of the Spanish National Research Council in Bellaterra and colleagues describe the possible new world in the April 6 issue of The Astrophysical Journal.

Ribas and colleagues say the planet, dubbed GJ 436c, orbits a red dwarf star about 30 light-years away in the constellation Leo.

According to the study team, GJ 436c would also be the first exoplanet revealed by its affect on the orbit of a planetary neighbor.

"Because of this," Ribas said, "the study opens a new path that should lead to the discovery of even smaller planets in the near future, with the goal of eventually finding worlds more and more similar to the Earth."

Four-Year-Long Day

Ribas and colleagues predicted GJ 436c after noticing that GJ 436b, the known Neptune-like exoplanet, passed in front of its host star in 2007. Observations in 2004 had not revealed such a transit.

Computer models of the system therefore suggested that another world was affecting the gas giant's orbit.

The models predict that the new planet is a rocky type with a radius some 50 percent larger than Earth's.

Temperatures on the planet range between 260 and 800 degrees Fahrenheit (127 and 427 degrees Celsius), the team calculated.

"But it could locally be as low as 170 degrees Fahrenheit (77 degrees Celsius) at the poles, depending on the type of atmosphere," team member Giovanna Tinetti of University College London said in a press release.

GJ 436c appears to make an orbit around its host star in only 5.2 Earth days, compared to Earth's roughly 365-day orbit. And unlike Earth's 24-hour rotation, the planet makes a full revolution on its spin axis in 4.2 Earth days.

This means that a full day on the new planet would take four planetary years, or roughly 22 Earth days.

Stephane Udry, of the Observatory of Geneva in Switzerland, announced the discovery of the potentially habitable super-Earth Gl 581c last year.

He said the new planet, if confirmed, would be another piece to an exciting puzzle.

"I am completely convinced that habitable rocky worlds around stars similar to the sun will be discovered in a close future," he said. "We are living in an exciting time."

But he's not convinced yet that the latest super-Earth is real, because of conflicts in previous data.

"For me the planet's existence, although possible, is still academic."

Wobble and Bend

Most of the 280-plus known exoplanets have been found using a technique called Doppler wobble, which detects planets' gravitational tugs on their parent stars.

The wobble method most easily detects gas giants similar to the mass of Jupiter, although some with masses below ten times that of Earth's have also been found.

Several super-Earths also have been revealed by Doppler wobble.

Jean-Philippe Beaulieu, a visiting astrophysicist at University College London, called GJ 436c the "hot twin" of the frozen super-Earth OGLE-2005-BLG-390Lb that he and colleagues discovered two years ago.

That similarly sized world was found using gravitational microlensing, a method that looks for objects between Earth and a star by detecting how light from the star bends.

He says the new find provides an important new technique for detecting smaller terrestrial exoplanets.

"It is important to build up a large zoo of extrasolar planets, to understand their properties," Beaulieu said.

"Soon one of them will be [found] in the habitable zone"—the region around a star where liquid water is able to exist.

But the authors agree that more confirmation is needed to prove the existence of GJ 436c.

"The story of the planets orbiting this red dwarf [GJ 436] is not yet completely written," Beaulieu said. "More fun should be expected in the coming months."

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