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Smallest exoplanet found

Although this exoplanet is the most earthlike extrasolar body yet detected, it is quite different from our home.
Earthlike exoplanet
Lynette Cook
Earthlike exoplanet
In this artist's rendition, the newly discovered planet is shown as a hot, rocky, geologically-active world glowing in the deep red light of its nearby parent star, the M dwarf Gliese 876.
Trent Schindler, National Science Foundation
June 13, 2005
Astronomers have discovered more than 150 planets outside our solar system, ranging from 100 to 1,000 times Earth's mass. Today, astronomers announced the 155th exoplanet discovery. So far, this is the most earthlike planet found beyond the solar system.

At the National Science Foundation's headquarters in Arlington, Virginia, an ecstatic team spoke about the serendipitous discovery today. While carefully examining two Jupiter-size planets with the Keck Telescope in Hawaii, the team noticed a wobble that could not be accounted for by the two-planet model they used. Doppler measurements revealed an inner planet. It orbits Gliese 876, an M dwarf star about one-third of the Sun's mass.

"This planet answers an ancient question," says team leader Geoffrey Marcy. "Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other earthlike planets. Now, for the first time, we have evidence for a rocky planet around a normal star."

Little is known about the exoplanet, such as its chemical composition or terrain. Team members do recognize it possesses dissimilarities to Earth. First, it is larger than our home planet. Although astronomers believe it could be 6 to 9 times Earth's mass, the team estimates it is 7.5 times larger. Since previously discovered exoplanets are much larger — all bigger than Uranus, an ice-giant about 15 times Earth's mass — astronomers would place this body in an "earthlike class size."

The exoplanet's orbit around its star is much shorter than Earth's orbit around the Sun. The body makes one trip around Gliese 876 in only 1.94 days. It is located about 2 million miles (3.2 million kilometers) from its star, compared to Earth's position about 93 million miles (150 million km) away.

Furthermore, the exoplanet is much hotter than Earth — it's an oven, at 400º to 750º Fahrenheit (244º to 398º Celsius). While those who discovered the exoplanet believe these conditions could not support life, they have not ruled out the existence of water. The hot conditions also make it likely that the planet has not retained much gas, making the planet solid.

"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," says team member Steven Vogt.

Team member Jack Lissauer admitted this exoplanet won't hold the title of "most earthlike" for long. Astronomers will study this model and search similar star-class systems for even smaller exoplanets. Scheduled for launch in 2007, NASA's Kepler mission is designed to detect earth-size exoplanets.
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