The first two decades of exoplanet research yielded thousands of new worlds. The next two might reveal Earth twins, or even alien life.
Concept art of an exoplanet. Image: IAU/L. Calçada
The title of most "Earthlike" exoplanet has been in constant flux recently. Every few months, a new extrasolar planet waltzes in and upstages the last contender. Last year, it was Kepler-186f. Then it was Kepler-438b. Kepler-452b, which was announced by NASA with much fanfare on July 23, is now the popular favorite.
But what is lost in some of the public frenzy over these exoplanets is the fact that, well, they aren't that Earthlike at all. "The word 'Earthlike' is used way too often," said Sara Seager, a prolific exoplanet expert based at MIT, in a phone interview with Motherboard.
"In the media, if you look, an 'Earthlike' planet has been found again and again," she said. "No, it hasn't been found at all. You could call something 'the most Earthlike world ever found.' That is true. But the reality is that none of them are actually Earthlike."
For example, Kepler-452b, the exoplanet-du-jour, orbits a G2-class star like the Sun at around the same distance as Earth's orbit. Its orbital period is 385 days, which is strikingly similar to our own year, and it remains the smallest exoplanet ever found in the habitable zone of a Sunlike star.
That said, this planet is still estimated to be about 60 percent larger than Earth in diameter, and at least five times more massive. Those dimensions make a literal world of difference. Based on what scientists know about Kepler-452b, it is unclear if this planet is even rocky. It could also be a gas planet. The odds are roughly 50-50.
Indeed, though Kepler-452b was touted as "Earth 2.0" by NASA, it ranks all the way down at number six on the Earth Similarity Index (ESI). Kepler-452b has been a favorite because of its orbital period and distance from a Sunlike star, but if you widen the aperture of stellar classes to include smaller stars, Kepler-438b takes the lead on the ESI. Though this exoplanet orbits a red dwarf star once every 35 days, it is much closer to Earth in size and estimated surface temperature than Kepler-452b (though it does, regrettably, seem to be occasionally torched by solar flares).
In the end, it all depends on what parameters are considered to be most "Earthlike." If you choose to go by how similar the planet's entire solar system is to our own, planets like Kepler-452b will win out. But if you go by the individual characteristics of the planet itself—for instance, its mass, estimated temperature, and composition—planets like Kepler-438b take the lead. As exoplanet research continues to mature, these metrics are bound to complexify with it.
Of course, this is not intended to undermine the gains that exoplanet hunters have been making in the search for Earthlike worlds. Just the opposite. Over the past two decades, thousands of extrasolar planets have been discovered, most of which were detected as they passed in front of their stars, producing miniscule dips in stellar brightness.
This technique, called the transit method, hinges on planets being fortuitously aligned between their stars and our perspective here on Earth. Considering exoplanet researchers have found such a wealth of alien worlds using the transit method, which is based largely on luck, it boggles the mind to think what else might be out there, including planets like ours.
"There are so many planets," Seager told me. "Almost all of them have something fascinating about them. There are hot Jupiters, and hot Super-Earths, with surfaces that might be hot enough to melt rock. We have found planets that are so low density we don't even know what they're made of. They could be water worlds, like scaled-up versions of one of Jupiter's icy moons, that would be 50 percent water by mass."
"There's just so many things out there. I think the real excitement is in the huge diversity and all the different options that nobody has ever conceived of."
What's more, exoplanet research has itself diversified into dozens of subfields, each of which is tackling different aspects of the larger effort to find and characterize as many planets as possible.
As a result, the technologies involved in the field are rapidly evolving, with several next-generation observatories slated to come online over the next few years, including spacecraft like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) and massive ground telescopes, such as the controversial Thirty Meter Telescope (TMT).
"The only problem right now is that there are so many things that are happening, have happened, or are being planned, that it's impossible to simplify it," Seager told me.
Despite this kaleidoscopic quality, the search for habitable worlds remains at the forefront of the field, and Seager is among the many exoplanet experts who are finding creative ways to root out and study potentially life-bearing planets.
Her particular specialty is scanning exoplanetary atmospheres for "biosignature gases" that might have been produced by lifeforms. This is a longstanding and widespread angle for exoplanet researchers, but Seager and her team at MIT are taking it to a new and exhaustive level.
"In the field of biosignature gases, people have worked on oxygen, ozone, methane, and nitrous oxide [among other gases], but my group decided that any gas could be a biosignature gas because we don't know what kind of life is on another planet," she said. "We know that on Earth, thousands of gases are produced by life. But most of them aren't produced in high enough abundance to register in any meaningful way if, for example, aliens are looking at us from the type of telescopes we hope to build."
To counter that problem, Seager and her colleagues are compiling a massive list of thousands of candidate gases to build a more complex framework of assessing an exoplanet's probability of bearing life. She likened the idea to smelling out a skunk, in that one tiny dose of a potent substance can provide enough data to draw a conclusion.
"We're working through [the biosignature gas list] to say which one could be like that skunk smell which, even if there's a small amount, lasts long enough in the atmosphere and is strong enough to detect from far away," Seager said. "Eventually we'll try to find how we can disentangle it from other effects in that planet or in the atmosphere."
"I'm excited about it because independent of planets, it's just a cool Big Data project. We've been working through this, and finding a lot of interesting things about chemistry."
As these new approaches come to fruition over the coming years, genuinely Earthlike worlds will likely begin to fill out the list of known exoplanets, some of which may even have biosignature gases in their atmospheres.
But beyond looking for worlds with Earth's exact specs, this line of research stands to broaden the search for life-bearing worlds as a whole. Given that Earth is the only place we've ever observed life, it makes perfect sense to look for similar worlds, but as Seager notes, that doesn't exclude vastly different exoplanets from hosting life.
"In the next ten years, our chance to find a planet with a sign of life is very small, but we don't want to leave any stone unturned," she noted. "We would hate to find a planet, see something unusual in the atmosphere, and not recognize it. That would be a tragedy. So, we're doing everything we can to make sure that we absolutely maximize our chance to find life on another world."
Whether the first glimpse of potential alien life is more likely to come from an Earthlike planet or somewhere completely unexpected is anyone's guess at this juncture. But the good news is that our odds of finding it get better with each passing year.