AUSTIN — Astronomers have announced the detection of a system of three candidate exoplanets, each with a radius smaller than Earth’s. The planetary system’s host star, dubbed KOI-961, is a dim red dwarf in the NASA Kepler mission’s field of view. The planets are among the smallest discovered around a Sun-like star and is the most compact system to date. They are “an incredible find,” says Sara Seager (MIT), who was not involved with the study.The three bodies were found when they repeatedly crossed in front of KOI-961, creating minute dips in the star’s light. So far they’re still candidate planets because astronomers haven’t detected them using other means. The most common follow-up method measures wobbles in the star’s position along our line of sight caused by its planets as they orbit, from which researchers can calculate the planets’ masses. But KOI-961 has a visual magnitude of about 16, which is a couple of magnitudes too dim for precise wobble measurements using current instruments.
Still, the international team of astronomers was remarkably cautious in its analysis. The researchers spend most of their 13-page paper minutely considering the host star’s properties and various alternatives to the triple exoplanet result. And with such a high number of transit observations — several hundred crossings were observed for the innermost planet, says team member John Johnson (Caltech) — it’s highly unlikely the signals aren’t real.
The KOI-961 system came to the researchers’ attention through a lucky break, Johnson explains. One of Johnson’s collaborators is a British amateur astronomer named Kevin Apps, who in his spare time “devours” astronomy research papers and memorizes catalogs, which he can compare in his head. It was Apps who alerted Johnson’s team to the fact that KOI-961 had properties eerily similar to those of Barnard’s Star, one of the nearest and best-studied stars in the sky.“It was as if it was the exact same star,” Johnson says. Using Barnard’s Star as a base, the astronomers determined KOI-961’s properties to high precision. From those estimates they found sizes for the three planets: 73, 78, and 57 percent of Earth’s average girth, from nearest to farthest from the star. The smallest, KOI-961.03, is about the same size as Mars. None resides in KOI-961’s habitable zone.
Without mass measurements the astronomers don’t know the planets’ densities, so they can’t say what they’re made of. But common sense dictates they’re probably rocky, Johnson says. Even if the planets were made completely of iron (which is improbable at best), they would still be less than two Earth masses.
The new exoplanets have raised hopes of detecting planets around more dwarf stars, allowing astronomers to better understand how many of these small stars have families of their own. “It’s kind of like cockroaches,” Johnson says. “If you see one…”