All this week, The Takeaway is exploring distant worlds light years away from Earth in our new series, “Brave New Worlds: Looking for Life in The Goldilocks Zone.”
Of the hundreds of planets that the Kepler Spacecraft has confirmed, only a handful have been considered very "Earth-like." One of these planets is Kepler-438b, about 470 light years from Earth.
What would it take for Kepler-438b to be habitable? For answers, we turn to Dr. Natalie Batalha, an astronomer at NASA’s Ames Research Center and a Kepler Mission scientist.
“At first glance, this appears to be a planet very much like Earth,” says Batalha. “It’s about the same size—it’s only about 10 percent larger than the Earth in its radius. It receives about the same amount of energy from its star that Earth receives from its sun. So, you might think that it’s going to be very Earth-like, but as we travel there and take a closer look, we start to notice some big differences.”
For starters, Batalha says that Kepler-438b's star is very different from our sun—it's an M-class dwarf star that's small, dim, and ruddy. It's these properties that dictate the conditions on Kepler-438b.
“It’s about half the size of our sun and it’s about 2,000 degrees cooler than our own sun,” she says. “It’s emitting mostly red and infrared radiation as opposed to the yellow, visible light that our own sun emits. As we’re flying towards it, if you can imagine that, we won’t see it as being this really deep red star—we’d see kind of an orangish, yellow hue just because of the response from our eyeball and how that’s evolved on our own Earth.”
Batalha says M-class dwarf stars have intense magnetic fields that give rise to sunspots and coronal mass ejections—those are massive bursts of magnetically charged gasses similar to what we can see from our own sun. But with M-class dwarf stars, things are exaggerated tenfold.
If a human were to theoretically stand on the surface Kepler-438b, a viewer would see a spectacular light show in the sky because of the intensity of the planet’s star.
“If [Kepler-438b] itself has magnetic fields like our own Earth does, then the wind of charged particles that’s coming out of these coronal mass ejections are going to slam into the planet but be funneled by the magnetic fields towards the poles,” says Batalha. “When these charged particles get funneled, they start colliding with the atmosphere. That creates an emission that we know [on Earth] as the northern and southern lights.”
Though it might be beautiful, Batalha says that if humans were to theoretically homestead on Kepler-438b, rotation would be necessary to generate magnetic fields. But that may not be an option 470 light years away.
“What we know is that all life is carbon-based, and carbon-based chemistry requires water for its survival,” she says. “We’re looking for planets where water can pool on the surface. But if you’ve got a central start with such a low luminosity—only four percent of the sun’s luminosity—you’re going to have to cozy up close to that star to create those conditions.”
Because of the size and dimness of an M-class dwarf star, Kepler-438b or any potentially habitable planet would need to be very close in order to get the right amount of energy to have liquid water on the surface, subjecting it to strong tidal forces.
Batalha says that's likely to lock the planet into a synchronous rotation, meaning only one side of the planet ever faces the star—something that’s similar to the way Earth's moon faces our planet. And that could create some very alien environments on Kepler-438b.
“The Earth is kind of like a rotisserie chicken—it’s spinning on its axis and it gets nicely toasted on all sides,” she says. “If you have a planet that’s locked in a synchronous rotation, than that rotisserie chicken is only getting cooked on one side. But it’s not quite so dire. Kepler-438b has an atmosphere. That atmosphere can be heated on one side, but then it’s free to circulate through these extreme temperature gradients to the other side. You could end up with a more temperate situation where the atmosphere is redistributing from the day side over to the night side, thereby making it more amenable to life.”
Batalha says it’s likely that Kepler-438b didn’t always have a synchronous rotation, but it’s unknown how long it might it’s been that way.
“I would think that, with time, this kind of dramatic weather, just like in a convection oven where you’re circulating air, you’re going to set up tremendous circulation patterns—global circulation or convention cells that are going to create tremendous winds,” she says. “You might imagine, initially, that you’d heat up the surface at that substellar point dramatically, and if there were oceans there you might evaporate a large quantity of water, which will create huge, thick clouds.”
Initially, the clouds on Kepler-438b might work to cool the surface of the planet, blocking the heat and radiation from its star. But that model would not be sustainable in the long term.
“You’d be kind of OK for a while, but it would continue to heat,” says Batalha. “You would start getting that circulation and you’d push that warm air over to the back side. And then what’s going to happen? It’s going condense, rain out—maybe even freeze. With time, slowly but surely, what I imagine might happen is the oceans on the dayside might gradually evaporate away and you would start piling up all of that water in a frozen state on the backside.”
Though building a life in the “sweet spot” between the dark side and light side might be a human’s best bet, it seems that Kepler-438b is no home-sweet-home.
“For us, if we were to be airlifted in to Kepler-438b, we’d have a hard time,” says Batalha. “But you know, life is prolific, robust and creative, and I could imagine that there would be life on that planet that’s adapted just fine.”