In good news for future efforts to colonize other worlds, and perhaps also find life in our galactic neighborhood, it seems we have been overlooking almost half the nearby Earth-sized planets. Whether the missing planets' circumstances present a major obstacle to life there is still an open question, however.
The Kepler and TESS planet-hunting space telescopes have turned the dozens of planets beyond the Solar System we were previously aware of into thousands. However, even with eyes in space it remains easier to detect larger worlds than smaller ones, so the sample is biased towards gas giants.
According to a paper accepted by the Astronomical Journal (preprint on ArXiv.org) it's even more skewed than we knew because these telescopes frequently miss Earth-sized planets in binary star systems. Since more than half the stars in the galaxy come in pairs or more, that's a large portion of the pool of rocky planets to be overlooking.
Many binary stars are in such tight orbits we don't even realize there are two stars there without close examination, and that's not something TESS is well suited to checking. Dr Kathryn Lester of NASA's Ames Research Center and colleagues used some of the largest telescopes on Earth to examine 517 of these stars, not for planets, but for overlooked stellar companions.
“With the Gemini Observatory's 8.1-meter telescopes, we obtained extremely high-resolution images of exoplanet host stars and detected stellar companions at very small separations," Lester said in a statement.
Lester and team found 102 binaries in the sample. TESS has already found candidate planets in these systems by observing the dips in brightness as one or other of the stars is blocked by an orbiting object. However, when Lester looked at the list of TESS's discoveries something stood out – almost all the finds were gas giants, unlike the mix of planet sizes found orbiting single stars. In fact, some are so large they probably aren't planets at all, but faint red dwarf stars, making these systems triples, rather than binaries.
It's possible this reflects greater difficulties in rocky planets forming within binary systems. Indeed, the stars in question have wider separations than a random binary sample, suggesting planets of any kind get disrupted when the stars are too close together.
Nevertheless, Lester thinks many Earth-like planets are out there, but too hard for TESS to see. "We have shown that it is more difficult to find Earth-sized planets in binary systems because small planets get lost in the glare of their two parent stars," Lester said. Her co-author, Professor Steve Howell, added: "Their transits are 'filled in' by the light from the companion star.”
Finding planets in these star systems will take additional observations, and possibly different techniques, the authors say. The stars Lester and Howell looked at are up to 1,600 light-years away, but patterns found in studying them could have relevance for nearby stars. In particular, it might tell us why we haven't found planets in the Alpha Centauri system, our closest star system and planetary system. A discovery, particularly of an Earth-sized world within this nearby binary would increase the interest in plans to build launch systems capable of sending probes there in the near future.
Whether life is possible for planets with two suns, unless they are so far apart one is little more than a bright star, remains debated. However, if we can reach them they could probably serve as staging posts for longer space journeys, and at least offer great sunsets.