Every week, it seems, astronomers announce a revolutionary exoplanet discovery — two plausibly habitable worlds orbiting a single star, for example, or a single planet orbiting two stars, or a planet no bigger than our moon or an exoplanet right next door in the Alpha Centauri system. Each one is important in its own way, but scientists also know focusing exclusively on single discoveries is like looking at the individual pixels on a screen and ignoring the larger picture they’re painting. Now and again, therefore, they take a step back and consider what they can learn from everything they’ve found so far. The result can be a real eye-opener.
That kind of big-picture consideration just happened again, with the publication of two papers in Science that assess the state of current knowledge about exoplanets. The first, by the University of Hawaii’s Andrew Howard, highlights an entirely new class of planets, midway in size between Earth and Neptune, that nobody had predicted — and which turn out to be incredibly common in the Milky Way. And the second, a theoretical paper by Sara Seager at MIT, argues that our definition of what makes a world habitable may be far too restrictive. Planets that would ordinarily be dismissed as too big or too small, too near their stars or too far away to be life-friendly might be able host living organisms after all.
The abundance of the midsize planets — what some people call “Super Earths” and others call “Mini Neptunes” — has emerged from several different planet-hunting surveys, but especially from the Kepler mission. The space-based Kepler telescope has been staring at a single patch of sky for four years now, waiting for stars to wink as planets pass in front of them. The smaller the planet, the longer it takes to be sure the wink is real, not just a flicker of the star or some other glitch.
By now, however, Kepler’s unblinking eye has found nearly 3,000 candidate planets, and when you chart them by size, they get more and more numerous as you go down the scale. They then hit a plateau at this new, unexpected size. “For me,” says Howard, “it’s kind of amazing that we keep expecting to find planetary systems like our own, and they keep turning out to be different.”
It’s not just the blink method that has helped reveal all the new, medium worlds. Also put to work has been the so-called “radial velocity” method, which looks for the wobbles an orbiting planet’s gravity induces in its star. When a star winks, you can gauge a planet’s physical size; when it wobbles, you can measure its mass. And when you spot the same planet with both techniques, you can combine those measurements to calculate density.
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That leads to another surprise: planets of about the same size come in significantly different masses, which means their composition is different — like a softball made of either lead or snow. “There are a lot of ways to make a planet twice the size of Earth,” says Howard. You can just scale up the Earth to form a big, rocky planet; or scale down Neptune to make a planet with a rocky core surrounded by a thick layer of water, surrounded in turn by a hydrogen-rich atmosphere; or you can make a planet that’s half rock, half water. “The range of densities we’re seeing wasn’t something most people were talking about beforehand, although it’s not that surprising in hindsight,” he says.
It also makes for a scenario that could be frustrating. “If Kepler finds a planet just the size of Earth in the habitable zone of its star,” says Howard, which is likely to happen within the next year or two, “we may not know [if they can’t measure the star’s wobble] if it’s made mostly of rock or whether it’s a little bit of rock with a huge atmosphere.” It could, in short, be Earth-size and in a life-friendly, Earth-like orbit and still be totally inhospitable.
But while Howard’s scenario may seem pessimistic, Seager’s pushes the hope meter back into positive territory. Whether a planet’s surface is the right temperature to allow water to exist in liquid form — a generally recognized requirement for life — depends only partly on how far it orbits from the warmth of its star. Another factor is its atmosphere and in particular the concentration of heat-trapping greenhouse gases it contains.
Usually, planetary scientists think of water vapor and carbon dioxide as the principal greenhouse gases, and here on Earth they are. But hydrogen is also a greenhouse gas. “It’s nasty,” says Seager, a pioneer in the theoretical study of planetary atmospheres. “It’s the worst.” But it’s also the best if your planet happens to be far from its star. A planet with a hydrogen-rich atmosphere, says Seager, could be 10 times farther out than Earth is and still be hospitable. In fact, she writes, hydrogen is so good a heat trapper that it might allow water to remain liquid on “rogue planets that were ejected from their birth planetary system and are now floating through the galaxy.”
Yet another sort of atmosphere would let an exoplanet stay hospitable much closer to its star than we are. Our water-vapor-rich atmosphere would make Earth far too hot — and in fact, Venus’ water-vapor-fueled runaway greenhouse effect is what turned that planet into an 800°F (427°C) hellscape. On a much drier planet than Earth or the young Venus, however, there would be a lot less water vapor and less trapped heat. “Not everyone agrees,” says Seager. “They don’t like dry planets.” But those are theoretical arguments, she says. “The main point I wanted to get across is that just about anything is possible. We won’t really know until we go out and observe.”
Thanks to Kepler and other existing planet searches — and in coming years new missions like the James Webb Space Telescope and NASA’s newly green-lit Transiting Exoplanet Survey Satellite — those observations will keep coming. Somewhere among them will be signs of a world that can — and perhaps does — harbor life.