Three super-Earths found around one star

By Jeanna Bryner

Senior writer

updated 11:54 a.m. ET June 16, 2008

A trio of planets called super-Earths has been spotted orbiting a sunlike star, astrophysicists announced Monday at an international conference in France.

Super-Earths are more massive than Earth but less massive than Uranus and Neptune. Spotting true Earth-sized planets is challenging with current technology, but the presence of super-Earths suggests finding a world like ours is just a matter of time, researchers say.

The team located the trio with the HARPS instrument on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. They inferred the existence of the planets by noting the worlds' gravitational affects on the parent star's orbit. This method is called the radial velocity, or wobble, technique.

In addition, HARPS astronomers have tallied about 45 new candidate planets with a mass below 30 Earth masses and an orbital period shorter than 50 days. The researchers say the deluge implies that one out of every three sunlike stars harbors such planets.

The trio's host star, HD 40307, is slightly less massive than the sun, and is located 42 light-years away, toward the southern Doradus and Pictor constellations. (A light-year is the distance light travels in one year, or about 5.88 trillion miles — 9.46 trillion kilometers.)

"We have made very precise measurements of the velocity of the star HD 40307 over the last five years, which clearly reveal the presence of three planets," said team member Michel Mayor of the Geneva Observatory in Switzerland.

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The smallest of the trio weighs in at 4.2 Earth masses and orbits HD 40307 every 4.3 Earth days, while the largest, with a mass 9.4 times that of Earth, has a 20.4-day orbit. The middleweight is 6.7 Earth masses and makes a 9.6-day-long trek around the star.

Since Mayor's 1995 discovery of a planet around the star 51 Pegasi, astronomers have noted more than 270 extrasolar planets, mostly around sunlike stars. Most of these planets are gas giants called "hot Jupiters." The researchers say about one out of every 14 stars outside our solar system harbors a hot Jupiter.

A basketful of other new exoplanets also got the spotlight at the same international conference, where researchers focused on extra-solar super-Earths. These included:

• A duo orbiting the star HD 181433: a super-Earth (7.5 Earth masses) that orbits its star every 9.5 days, and a Jupiterlike planet with a nearly three-year period.
• Another planetary system that has a 22 Earth-mass planet having a period of four days, and a Saturnlike planet with a three-year period.

"It is most probable that there are many other planets present: not only super-Earth and Neptunelike planets with longer periods, but also Earthlike planets that we cannot detect yet," team member Stephane Udry, also of the Geneva Observatory, said in an ESO statement. "Add to it the Jupiterlike planets already known, and you may well arrive at the conclusion that planets are ubiquitous."

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