Why This Earth-Like Planet is a Big Step Forward in the Search for Life

Jeffrey Kluger

If planets were products their price would be tumbling. Little more than a generation ago, we knew of only eight planets in all the universe—the ones within our own solar system. Only two of them, Earth and Mars, were plausibly capable of supporting life and only one of those definitely does. Planetary demand far exceeded supply.

Now, however, the market has been flooded. Thanks to advances in ground-and space-based observatories, especially the Kepler Space Telescope, which was launched in 2009 and operated for nine years, the population of known exoplanets — or planets orbiting other stars — has exploded to more than 4,000, with about another 4,000 detected but yet to be confirmed. Virtually every star in the universe is thought to be home to at least one planet, with some hosting an entire litter.

But a big sample group does not mean that science has yet discovered the true jackpot world: an Earth-like planet with a solid surface, an atmosphere and liquid water. If you’re looking for life, that’s where you’re likeliest to find it. Now, that jackpot—minus the life (so far)—appears to have been hit. According to a study by a team of researchers from the Center for Planetary Sciences at the University College London (UCL) and published today in Nature Astronomy, a potential garden planet, going by the prosaic name K2-18b, has been found just 110 light years from Earth.

“From today onwards, we know K2-18b has atmosphere and water, making it the best known candidate for habitability,” says Angelos Tsiaris, a UCL research associate in the Department of Physics and Astronomy and the lead author of the Nature paper.

Discovered in 2015, K2-18b was one more of the thousands of planets discovered by Kepler. The telescope did its work not by imaging its target planets directly, but rather by detecting the slight dip in light whenever the planet orbited in front of the Earth-facing side of its star. The frequency of the dip tells you how long each orbit takes and the amount of the dimming tells you the planet’s diameter—the bigger the world the more light is briefly lost.

Using those techniques, Kepler scientists determined that K2-18b is about twice the diameter of Earth and zips through its orbit once every 33 days. Two years later, European astronomers used a ground-based telescope to measure the amount of wobble K2-18b causes in its planet star as it orbits, which reveals the planet’s mass; the greater the wobble the more massive the world. Using those findings they concluded that K2-18b weighs in at about 8 times Earth’s mass.

That combination of diameter and mass put the planet in the category known as super-Earths — bigger than our comparatively small world; smaller than gas giants like Jupiter and Neptune; and likely to have a solid, rocky surface. In terms of life, that’s a good start. But K2-18b’s zippy orbit could also present problems. In order to complete a revolution so fast, the planet had to be situated very close to its sun—just 13 million miles away, the Kepler scientists calculated. Earth, by comparison, is 93 million miles from our sun. Mercury is just 36 million — giving it a surface temperature of 800º F (427º C).

Proximity is not a problem for K2-18b, however. Its sun is not a hot yellow star like ours, but a smaller, cooler red dwarf. That means its so-called habitable zone — the distance at which surface temperatures on the planet are within the narrow range to allow liquid water to exist — would be much closer. Correcting for the cooler fires of a red dwarf, astronomers conclude that even at so close a remove, K2-18b has surface temperatures that range from a low of -100º F (-73º C) to a high of 116º F (47º C).

Still, the question remained: Does the liquid water that could exist there actually exist there? To get the answer, the UCL team turned to one of the great workhorses of modern astronomy, the Hubble Space Telescope. Launched in 1990, Hubble predates the discovery of the very first exoplanet by two years, but its age is no bar to its ongoing productivity. The UCL researchers made use of the telescope’s Wide Field Camera 3, an instrument that can see in visible wavelengths, as well as in ultraviolet and near-infrared. What they were looking for were chemical signatures of K2-18b’s atmosphere—both whether the planet has an atmosphere at all, and whether that atmosphere contains water. They knew the work would be challenging.

“The measurements involved were extremely difficult,” says Tsiaris. “Like trying to identify one person in a crowd of ten thousand.”

But the precision of the Hubble and the tenacity of the researchers were up to the task, and over the course of multiple observations, they concluded that not only does the planet have water, it seems to have lots of it—making up as much as 50 percent of the atmosphere. It took a long time, however, before they were confident enough in their findings to announce them to the world.

“I had the results a year ago,” Tsiaris says. “At first we weren’t sure exactly what it meant, but we knew we had something exciting. It took us a year of repeating data analyses to make sure that what we say we found was correct. To get you to these two facts—there is atmosphere and there is water—took a lot of effort.”

K2-18b will surely be receiving more attention from astronomers around the world, not only with existing telescopes but with next-generation ones, including NASA’s James Webb Space Telescope, tentatively set for launch in 2021; and the European Space Agency’s ARIEL space telescope, specifically designed to study the atmosphere of exoplanets, and targeted for a 2028 launch. Already though, the K2-18b findings are encouraging more investigations of potentially similar worlds.

“If planets like Earth are very common,” says Tsiaris, “we can say life is very common.” That, of course, is an assumption that has yet to be proven, but with today’s announcement, the proof is closer than it’s ever been. —With reporting by Maddy Roache/London