Otherworldly triple play

Astronomers have discovered the first known system of three superEarths beyond the solar system

NANTES, France — Astronomers have discovered the first known system around a distant star with three planets categorized as superEarths.

SUPEREARTHS An artist’s depiction of three new exoplanets orbiting a star called HD 40307. ESO

The three extrasolar planets, all presumably rocky, range in mass from 4.2 to 9.4 times that of Earth. They orbit a star called HD 40307 just 42 light-years from Earth. HD 40307 has a mass about 80 percent that of the sun.

Although the planets all lie too close to their parent star to support life, the finding bodes well for ultimately finding a true twin of our planet, says Stephane Udry of Geneva Observatory in Switzerland, a member of the discovery team. Veteran planet hunter Michel Mayor of Geneva Observatory announced the discovery on June 16 at a conference on superEarths in Nantes, France.

Combined with four other planets, including another superEarth that Mayor’s team also announced at the conference, the new find brings the number of extrasolar orbs now known to more than 300.

The triple superEarth system was detected by a now-standard technique: looking for tiny, telltale wobbles in the motion of a star that reveal the gravitational tug of unseen, orbiting planets. That technique has found dozens of giant, Jupiter-mass planets. But because the planets in the new system are about one-hundred-thousandth the size of their parent star, the wobbles they induce are much tinier and more difficult to measure. Each planet induces a leisurely, tiny wobble — a back-and-forth motion in HD 40307 of about 2 meters per second, the pace of a person walking briskly.

Over a period of five years, Mayor’s team used the High Accuracy Radial Velocity Planet Searcher at the European Southern Observatory in La Silla, Chile, to study the motion of the star and reveal the presence of the unseen planets. Going from the innermost planet to the outermost, the planets are 4.2, 6.8 and 9.4 times the mass of Earth and whip about their star in circular orbits that take 4.3, 9.6 and 20.5 days.

“The detection of a triple-SuperEarth system is monumental,” says extrasolar planet observer David Charbonneau of the Harvard-SmithsonianCenter for Astrophysics in Cambridge, Mass. “First, it demonstrates the remarkable advances in the measurement precision achieved by these cutting-edge instruments. Second, the detection of three presumably rocky planets orbiting a single star is historic since that system really is reminiscent of our own solar system.”

In addition, the finding, along with the results of a survey of some 200 stars studied by Mayor’s team, suggests that a whopping 30 percent of sunlike stars have relatively small, close-in planets, ranging from 4 to 30 times the mass of Earth. That result was presented at a conference in May (SN Online: Many stars, many planets 5/23/08).

“It is possible that all of the systems of relatively close-in gas giants that we have studied to date are actually the minority, and that most stars like the sun actually have systems of terrestrial planets,” says Charbonneau. “While this singular discovery in itself doesn’t prove this, it means we can now detect such objects and we should know the answer soon.”

Eight out of the nine known extrasolar planets that weigh less than 25 Earths — including the new discoveries — are part of multiple planet systems. That high percentage also appears to hold true for 45 other candidate low-mass planets that await confirmation, Mayor says.

In the standard picture of planet formation, gas and dust within a swirling disk surrounding a young star gather together and form larger and larger bodies — first planetesimals and finally full-fledged planets.

New theoretical models, including one developed by Doug Lin of the University of California, Santa Cruz and his colleagues, indicate that it may be relatively easy for rocky, low-mass planets to form and then escape from the disk, migrating inwards — the vicinity in which Mayor’s team and other researchers have found such planets. Lin and Shigeru Ida of the Tokyo Institute of Technology describe their work in an upcoming Astrophysical Journal.

In their work, Lin and Ida show that hot superEarths have a higher probability of forming around low-mass stars, such as HD 40307, than stars like the sun. That’s because superEarths can more easily migrate from the protoplanetary disk to the low-mass star, and can also park themselves in stable orbits close to the star. In some cases, a succession of superEarths can push each other from an outer island of stable orbits to a set of orbits closer to the star. There, the planets adopt circular paths because of the strong gravitational interaction with the parent star.

“The better we understand how superEarths form, the better we will know where to look for Earth-type planets,” Udry says. That’s becoming critical, he adds, because “we are getting close to the precision needed to find Earth-type planets in the habitable zone around sunlike stars.” Follow-up observations are likely to happen in a hurry. Udry says his team has just requested time on the infrared Spitzer Space Telescope to determine any of the superEarths among the trio might transit, or pass in front of its star as seen from Earth, periodically, creating a mini-eclipse.

“Should one of these [planets] be observed to transit, that would be wild,” says Charbonneau. ”We could [then] determine the mass and radius directly, and hence determine whether it truly is composed of rock like the Earth and hence is properly called a ‘superEarth.’” Researchers could also measure the planet’s spectrum and look for biomarkers, signs that the orbs might have some of the chemical composition suitable for life, he notes.

At the meeting in Nantes, Mayor’s team also announced the discovery of two other planetary systems using the same spectrograph. One system features a 7.5-Earth-mass planet in a 9.5-day orbit about the star HD 181433. That star also has a giant outer planet, about the mass of Jupiter, in a three-year orbit. The other system contains a planet slightly heavier than Neptune in a four-day orbit along with an outer, Saturn-mass planet in a three-year orbit. Udry says he and his collaborators have found another 15 or so planets that they will announce over the next few months.

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