Three “Super-Earths” Found Orbiting One Star

“Does every single star harbor planets and, if yes, how many?” wonders planet hunter Michel Mayor. “We may not yet know the answer but we are making huge progress towards it.” Mayor and his team of European astronomers have found a star which is orbited by at least three planets. Using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the ESO La Silla Observatory, they have found a triple system of super-Earths around the star HD 40307. This is the first system known to have at least three “super-Earth” sized planets.

Back in 1995, Mayor, along with Didier Queloz, made the first discovery of an extrasolar planet around 51 Pegasi, and since then more than 270 exoplanets have been found, mostly around sun-like stars.

Most of these planets are giants, such as Jupiter or Saturn, and current statistics show that about 1 out of 14 stars harbors this kind of planet.

“With the advent of much more precise instruments such as the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, we can now discover smaller planets, with masses between 2 and 10 times the Earth’s mass,” says Stéphane Udry, one of Mayor’s colleagues. Such planets are called super-Earths, as they are more massive than the Earth but less massive than Uranus and Neptune (about 15 Earth masses).

HD 40307 is slightly less massive than our Sun, and is located 42 light-years away towards the southern Doradus and Pictor constellations.

“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,” says Mayor.

The planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively.

The group made the announcement at a conference about extrasolar planets being held in France. The same team also announced the discovery of two other planetary systems, also with the HARPS spectrograph. In one, a super-Earth (7.5 Earth masses) orbits the star HD 181433 in 9.5 days. This star also hosts a Jupiter-like planet with a period close to 3 years. The second system contains a 22 Earth-mass planet having a period of 4 days and a Saturn-like planet with a 3-year period as well.

“Clearly these planets are only the tip of the iceberg,” says Mayor. “The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days.”

A planet in a tight, short-period orbit is indeed easier to find than one in a wide, long-period orbit.
“It is most probable that there are many other planets present: not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet. Add to it the Jupiter-like planets already known, and you may well arrive at the conclusion that planets are ubiquitous,” concludes Udry.

Calculations from the sample of stars studied with HARPS implies that one solar-like star out of three harbors planets with masses below 30 Earth masses and an orbital period shorter than 50 days.

News Source: ESO press release