Still no Earths, but getting closer

Only a few years ago, astronomers were thrilled if they found a star beyond the solar system harboring a single planet. Now they’re discovering more and more multiple-planet systems that may offer new clues about the formation of planets and their evolution.

In one new study, scientists have discovered a star with at least five and as many as seven planets, which would make it the richest known planetary retinue beyond the solar system. A second study has revealed a star with two Saturn-mass planets locked in a special gravitational embrace that allows astronomers to study the pair in unusual detail. Researchers also find hints of a third planet orbiting with the two. If confirmed, the planet would be the tiniest extrasolar orb known, with a diameter only 1.5 times that of Earth.

With these multiple-planet systems, “we’re entering a new era of exoplanets,” says theorist Sara Seager of MIT. Instead of focusing on individual discoveries, she adds, “we’re moving on to complex planetary system architectures and the hope of being able to understand how they got that way.”

Using NASA’s Kepler spacecraft, Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues found two planets orbiting a dim sunlike star dubbed Kepler 9, some 2,200 light-years from Earth. Each planet blocks a tiny amount of starlight when it transits, or passes across the face of the star.

This is the first time a system has been found with more than one transiting planet. What’s more, the two planets have migrated into orbits that have a special gravitational synchrony — every time the outer planet makes one lap around Kepler 9, the inner planet makes two, Holman and his collaborators report online August 26 in Science.

During seven months of observations, Kepler not only measured dips in starlight, which reveal the diameters of the planets, but also found that the transit time of each planet varied by minutes due to the orbs’ mutual gravitational tug.

Holman and his collaborators used the timing variations to determine that both planets have masses similar to Saturn, a finding confirmed by ground-based observations of the back-and-forth motion, or wobble, of the parent star. In the future, says Holman, variations in transit timing recorded by Kepler may reveal the masses of planets similar to Earth — bodies too lightweight to induce a detectable wobble in the orbit of their star.

A much smaller dip in starlight may be due to a third, tinier planet with a diameter only 1.5 times that of Earth orbiting Kepler 9. The candidate planet would lie much too close to the star to support life. Holman and his collaborators posted their analysis of the proposed “super-Earth” signal August 27 at arXiv.org.

If confirmed, the super-Earth would be the “smoking gun” for a leading theory of planet formation, comments theorist Doug Lin of the University of California, Santa Cruz. According to that theory, known as core accretion, bits of material from a disk of gas, dust and ice that swaddles newborn stars coalesce to form a solid core. Then the core may snare a massive amount of gas from the planet-forming disk to create a gas giant like Jupiter. If the gaseous disk disperses before that can happen, a rocky naked core more like Venus, Earth, Mars or Mercury remains (SN: 3/26/05, p. 203).

A rocky super-Earth orbiting closer to Kepler 9 than the gaseous Saturn-like planets do would be strong evidence of just such a naked core, pushed within roasting distance of the star by the two heavier planets, says Lin.

As intriguing as the Kepler 9 system appears, it’s downright simple compared with the gaggle of planets orbiting HD 10180, some 127 light-years from Earth. Tracking the motion of the star for six years with a sensitive spectrograph on the European Southern Observatory’s 3.6-meter telescope at La Silla, Chile, researchers found evidence for at least five planets, with hints of an additional two.

The five confirmed planets have minimum masses similar to that of Neptune and are packed inside a region roughly equal in diameter to Mars’ orbit around the sun, Christophe Lovis of the Geneva University Observatory in Switzerland and his colleagues report in an upcoming Astronomy & Astrophysics. The system is much more densely populated than the inner solar system and also appears to lack a Jupiter-sized planet.

Understanding how the HD 10180 system came to be poses challenges that promise to provide additional clues about planet formation and the conditions necessary to make and retain Earth-mass bodies, says Lin.

Back Story: Hunting Other Earths

Image credit: NASA.

Launched in 2009, the Kepler spacecraft regularly monitors over 100,000 sunlike stars looking for tiny, periodic dips in brightness (shown as a change in the light curve, see image) that indicate the passage of a planet across a star’s face. The frequency and magnitude of these planetary transits can be used to help deduce a distant world’s composition and chances of being habitable.