In Search For Habitable Planets, Why Stop At 'Earth-Like'?
In their hunt for potentially habitable planets around distant stars, scientists have been so focused on finding Earth-like planets that they're ignoring the possibility that other kinds of planets might be even friendlier to life, a new report says.
So-called superhabitable worlds wouldn't necessarily look like Earth but would nonetheless have conditions that are more suitable for life to emerge and evolve, according to the study published this month in the journal Astrobiology.
"In my point of view, astronomers and biologists are biased," says Rene Heller, an astrophysicist at Canada's McMaster University who is the study's lead author. "These scientists look for planets that are Earth-like."
But it's possible that Earth is actually only marginally habitable by the standards of the universe, says Heller, who points out that our home may not represent a typical habitable world.
He and co-author John Armstrong of Weber State University in Utah have come up with a long list of traits that might make a planet "superhabitable."
Such planets would most likely be older than Earth and two to three times bigger, the researchers say. And they would orbit stars that are somewhat less massive than our sun.
Any liquid water wouldn't be in a giant, deep ocean, but would be scattered over the surface of the planets in shallow reservoirs. The planets would need a global magnetic field to serve as protection from cosmic radiation, and they would probably have thicker atmospheres than the Earth does.
"It's good to start thinking now about how do we sort of rank these planets in terms of their potential to host life," agrees Rory Barnes of the University of Washington, who uses computer models to explore the habitability of planets outside our solar system. "I think this paper does a really good job of examining the different kinds of features that all come into play when making a habitable planet."
So far, scientists have detected about a thousand planets orbiting other stars. Current technology usually can't reveal much about them — just a planet's size, density and how far it orbits from its host star.
Planets are said to be in the "habitable zone" when their surface temperatures would potentially allow liquid water to exist on the surface.
"That's all we can really say at this point," Barnes says. "We don't have any clue if they actually are habitable, let alone if they are inhabited."
MELISSA BLOCK, HOST:
Scientists who study planets outside our solar system have long been hunting for Earth's twin. They want to find a small rocky planet just like our home because it would offer the right conditions for life as we know it. Well, now, a study suggests that other kinds of planets might offer even better conditions for life.
Here's NPR's Nell Greenfieldboyce.
NELL GREENFIELDBOYCE, BYLINE: Scientists know of one planet that is definitely habitable: Earth. But Rene Heller says if you're interested in finding other habitable planets that might harbor alien life, you shouldn't assume that the best strategy is to look for another Earth. Worlds that seem very different might be even more suitable for life to emerge and evolve.
RENE HELLER: Maybe there is a population of, as we term them, superhabitable planets, which are even more likely than Earth-like planets to actually be inhabited.
GREENFIELDBOYCE: Heller is an astrophysicist at McMaster University in Canada. He and a colleague have just published a new study in the journal Astrobiology that says we need to move beyond the idea that the Earth is the quintessential habitable planet. They list a bunch of features that could make a planet superhabitable. For example...
HELLER: These planets are likely more massive than Earth-like planets, and they will tend to be older than Earth.
GREENFIELDBOYCE: More massive means they'll probably have a better magnetic field to offer protection from radiation. Older means more time for life to take hold. And while the planet should have water, Heller says they wouldn't have one big deep ocean, but rather lots of shallow seas, because on Earth, shallow water is a fantastic place to live.
Rory Barnes is an astrobiologist at the University of Washington who uses computer models to explore the habitability of planets outside our solar system. He thinks the new report is a convincing list of the features that would make a planet an even more cozy home.
RORY BARNES: There are things about the Earth that we could improve upon. You know, we could imagine a world that would have more life on it or allow life to thrive for a longer period of time than our Earth.
GREENFIELDBOYCE: But being able to imagine that kind of world doesn't mean they can find it. So far, scientists have detected about a thousand planets orbiting other stars. Current technology usually can't reveal much, just a planet's size, density and how far it orbits from its host star.
BARNES: We have so little information that we can't really say that a planet is even habitable, which would mean maybe that it just has liquid water and no life. You know, we don't - we just can't get that information today.
GREENFIELDBOYCE: But Barnes says once scientists do have a list of planets that are potentially habitable, the ideas in this new report should help rank them in terms of the potential for life.
BARNES: And so it's sort of more forward-looking than I think what a lot of people are doing right now.
GREENFIELDBOYCE: Still, he does have this word of caution. It could very well be that some planets have conditions or processes that are great for life but so alien to us that we'd never think of them.
Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.