In 1989, scientists found the first evidence of planets outside our own familiar, little solar neighborhood.1 Today, the list of exoplanets — as measured by NASA’s Exoplanet Archive — has grown to 3,375. That includes the recently discovered Proxima Centauri b, a planet just four light-years away that could be habitable.
But potentially Earth-esque planets remain rare. The Planetary Habitability Laboratory database at the University of Puerto Rico’s Arecibo telescope facility counts just 10 exoplanets that are likely to be habitable.2 Ten, out of 3,375. That statistic creates an image of a universe filled with gas giants, flaming hot lava spheres and frozen snowballs — with nary a cozy, just-right second Earth to be found.
Appearances, however, can be deceiving. The list of habitable exoplanets is so short not because there aren’t many, scientists told me, but because the tools we use to find exoplanets are biased. The technology has been better at finding planets that aren’t much like Earth than it is at finding ones that are. The search for exoplanets has been less like a census of the stars and more like looking for stars with a Map of Stars’ Homes: You’re only seeing the really obvious stuff, and just because you don’t catch a glimpse of Jay-Z watering his lawn, it doesn’t mean he’s not there.
The vast majority of exoplanets we know about were discovered by a method called transit.
| DISCOVERY METHOD | PLANETS |
|---|---|
| Transit | 2,664 |
| Radial velocity | 593 |
| Imaging | 42 |
| Microlensing | 39 |
| Transit timing variations | 15 |
| Eclipse timing variations | 8 |
| Orbital brightness modulations | 6 |
| Pulsar timing variations | 5 |
| Pulsation timing variations | 2 |
| Astrometry | 1 |
With transit, scientists watch a star, measure its brightness and wait for that brightness to dim as a planet passes in front of it. It’s different from the radial velocity, or Doppler, technique — which was used to find Proxima Centauri b. There, scientists are looking for changes in the spectrum of light caused by interactions between the gravity of the star and the gravity of a nearby planet.
Both of these systems have limitations, scientists told me. The transit method can find so many more planets because it’s tracking only a single measurement: star brightness, said Jessie Christiansen, staff scientist at the NASA Exoplanet Science Institute. Kepler, the famous space-based telescope, can track brightness on 200,000 stars at once, she said. But transit can only spot planets that happen to geometrically line up in just the right way so that we see their star dim when they pass between it and Earth.
Radial velocity, meanwhile, is much more specific. Scientists separate the different wavelengths of light from stars so they can track how the spectrum changes and see the photonic echo of slight wobbles in the solar orbits. That means it’s harder to study faint, far-away stars — less of their light reaches us, to begin with — and scientists can look at only one star at a time, drastically slowing the rate of detection compared with transit. But radial velocity doesn’t depend on geometric good fortune, so it can see planets that transit never would. “That’s partly why people are so excited about the Proxima Centauri detection,” said Sara Seager, professor of planetary science and physics at MIT. “Radial velocity can find those, whereas if you were using transit technique, you might miss it because it wasn’t lined up.”
And both transit and radial velocity miss whole classes of planets, said Gibor Basri, professor emeritus of astronomy at the University of California, Berkeley. It’s always easier to find larger planets and planets that are closer to their star. “There’s almost no way to see, say, an Earth-sized planet that’s in Jupiter’s orbit,” he said.
Size matters. If you’d asked scientists what size was normal for exoplanets 10 years ago, the answer would have been: massive. “The first five years or so of having transiting planets, we were only finding these things we called ‘Hot Jupiters.’ Those are Jupiter-size things that go around their star every two or three days,” Christiansen said.
Since the Kepler telescope launched in 2009, though, that’s been reversed.
On this chart, you can see that, before that year, almost everything that had been found was more than 11 times the radius of Earth. “Before Kepler launched, 85 percent of the planet discoveries were larger than Neptune. Today, 85 percent of the discoveries are smaller than Neptune,” said Natalie Batalha, an astrophysicist and Kepler’s mission scientist. (Neptune’s radius is about four times that of Earth.)
Meanwhile, all of the exoplanets with the smallest radiuses — 1.25 the size of Earth’s or less — have been found since 2011.
The same is generally true of planet masses, as well. Most of the exoplanets closest to Earth’s mass have been found since 2008.
That’s thanks to Kepler, too — though indirectly. The transit method, and thus Kepler, can’t be used to calculate mass. But some of the planets Kepler found have been verified using other methods, such as radial velocity, which can.
But this brings us to what is probably the ultimate limitation. Most exoplanets haven’t been studied using both the transit and radial velocity methods. Transit can find a planet’s radius, but not mass. Radial velocity can find mass but not radius. And what scientists really, really want to know about is exoplanets’ density — a calculation that requires both mass and radius. “It’s like the holy grail for planets,” Christiansen said. That’s because density is what tells you if a planet is rocky (like Earth) or just a big ball of gas (like Jupiter). Density is how you really start to separate out the stuff that might be kind of Earth-ish, maybe, from the stuff that could be truly habitable.
This is why the fact that we’ve found 10 habitable exoplanets doesn’t mean there are only 10 habitable exoplanets. There could be way, way more than that. But our technology, and the way we’ve used it, hasn’t been optimized to find them.
That might be about to change. In late 2017 or early 2018, NASA plans to launch TESS — the Transiting Exoplanet Survey Satellite — which will pick up where Kepler left off and work alongside ground-based radial velocity telescopes. The goal: Use both techniques to find more densities for more planets. Even then, there will still be limitations. Some of the planets found by TESS and Kepler can’t be measured with radial velocity. They’re just too far away, and the signal would be too small for the current technology, Christiansen said. But, while it still won’t be a perfect census, TESS should, at least, expand our list of habitable worlds.