The most robust census yet suggests that they’re out there, but they might take time to locate.
Astronomers have found more than 4,000 planets circling distant stars, yet none feel quite like home. Teegarden b is the right size, but it zips around its dim dwarf star in just five (Earth) days. Kepler-452 b takes a familiar 385 days to complete an orbit around its sun-like star, but appears to be a lumbering “superterran” much more massive than the rock we call home. Where, or even whether, true Earth-twins exist remains one of astronomy’s top mysteries.
While today’s space telescopes lack the ideal skillsets for spotting an Earth 2.0, astronomers are starting to get a sense of how frequently similar worlds may pop up in the cosmos. By combining the final data sets from NASA’s exoplanet spotting spacecraft Kepler with other recent surveys, a team of astronomers has calculated the strongest such estimate yet: Visit somewhere between three and three dozen solar systems, they say, and you’ll likely come across at least one Earth. They hope their results will inform the design of upcoming exoplanet hunting telescopes, as well as our understanding of the odds of life as we know it existing elsewhere.
“Is there the possibility of other life out there in the universe?” asks Danley Tsu, a graduate student at Penn State and coauthor of the research. “Trying to estimate the frequency of Earth-like planets around sun-like stars is one of the ways we can answer that question.”
Spotting that one in a handful, however, is another matter.
NASA’s current exoplanet-seeking eye in the sky is the Transiting Exoplanet Survey Satellite (TESS), which searches for the tell-tale stellar dimming that indicates a planet has passed in front of its star. Its cameras sweep across a majority of the sky, prioritizing nearby solar systems close enough for the Hubble Space Telescope and upcoming James Webb Space Telescope (JWST) to take a closer look.
TESS has already found more than 1,000 potential (“candidate”) planets, and NASA expects it to find nearly 20,000 more. Of those, perhaps 500 will be Earth-sized, but almost none will be Earth-like. Astronomers have to spot three dimmings, or transits, to be sure they’re looking at an orbiting planet (rather than a random dust cloud or flicker), so TESS’s frequent scanning gives it time to find only planets that fly around their star in a matter of days or weeks—not years.
The satellite also targets cool, red dwarf stars. They far outnumber brighter “G-type” stars like our sun, making them a great focus for a huge exoplanet haul, but they may not make friendly homes for life. To orbit inside their so-called habitable zone, where planets get just enough energy to keep water from freezing or boiling, planets have to huddle right next to the star in what we might consider Mercury territory.
Calling these dwarf star Goldilocks zones “habitable,” however, smacks of optimism. Planets there might enjoy balmy temperatures, but the nearby star would also douse them with ultraviolet radiation and solar flares that could strip atmospheres and fry emerging microbes. Organisms may find ways to eke out survival, but they would have to get creative.
NASA’s previous flagship exoplanet-hunting spacecraft, Kepler, was tuned to brighter, sun-like stars. For years it stared unblinking at the same moon-sized patch of sky, collecting the light dips it needed to identify exoplanets. After about four years, however, just as Kepler had been watching long enough to begin to catch the third and fourth transits of stars taking hundreds of days to orbit, a part needed to keep it stable broke down.
“We unfortunately missed the window,” Tsu says, “because the spacecraft died right around when we were starting to get more and more Earth-like candidates.”
The result was a catalog full of diverse exoplanets, but not a single Earth 2.0. In the absence of hard observational evidence, astronomers turned to statistical tools to count the uncountable.
Chris Burke, an astronomer at MIT who worked previously on the Kepler mission and is now involved with TESS, likens the task to conducting a census. You count whoever you can and think very carefully about who you can’t reach and why. “Your census is never complete,” he says, “you have to understand where you’re missing people.”
In the case of Tsu and his collaborators, that meant an intimate understanding of the Kepler spacecraft’s strengths and weaknesses. Looking for a dimming star is theoretically simple, but in practice you have to worry about dead pixels, false alarms, binary stars masquerading as planets, how accurately you know the size of each star, and a litany of other complications. “Each one of these little things will shape those detections,” Burke says, and you have to learn how to figure out which planets got knocked out during the detection process and which were actually too hard to see.