Our Closest Earthlike Planet Appears to Be Covered in Water
Oceans on Proxima b could harbor life.
Jupiter's moon Europa recently became the best candidate yet for harboring alien life within our Solar System. But in the search for life outside of our cosmic backyard, an Earth-sized, newly-discovered rocky exoplanet has scientists woozy: They've concluded that the planet may in fact be covered in oceans of liquid water—water that could contain life.
In a study to be published in The Astrophysical Journal Letters, an international team led by researchers at the Marseille Astrophysics Laboratory (CNRS / Aix-Marseille Université) determined that the dimensions and properties of the surface of Proxima b "actually favor its habitability."
The researchers determined that the planet could be an "ocean planet with an ocean covering its entire surface," with water similar to that detected on the icy moons around Jupiter and Saturn such as Europa and Enceladus.
"The planet may very well host liquid water on its surface, and therefore also some forms of life," the researchers said, as cited by Agence Presse France.
"It is likely to harbor liquid water at its surface and therefore to harbor life forms"
Proxima b was discovered in August orbiting the star Proxima Centauri, 4.2 light years away from Earth. The planet is around 1.3 times the size of Earth, but orbits at a distance of 4.6 million miles from its sun, a tenth of the distance that our own Solar System's Mercury orbits at from our Sun.
For this reason, some scientists suggested that the planet would be too hot for liquid water, as its distance from Proxima Centauri would leave its oceans "tidally locked" and facing the star—left to heat up to uninhabitable temperatures or even boil off. But the researchers at CNRS have rebutted that idea, for now.
"Contrary to what one might expect, such proximity does not necessarily mean that Proxima b's surface is too hot for water to exist in liquid form," said the research team in a statement.
Proxima Centauri is about 12 percent the mass of our own Sun, and much dimmer. Hence, the habitable zone is shifted towards the star, meaning Proxima b could be nestled right where liquid water—and life—could propagate.
"As Proxima Centauri is a red dwarf, its mass and radius correspond to only one-tenth of the Sun, and its brightness is a thousand times smaller than our star. At such a distance, [Proxima b] is therefore in the habitable zone of its star. It is likely to harbor liquid water at its surface and therefore to harbor life forms," said the researchers.
But the astrophysicists at CNRS have still not been able to accurately determine the size of Proxima b, and the conclusions about liquid oceans are based from simulations of the planet's composition and radius. Typically, when working out the size of exoplanets, scientists measure how much light they block out when passing in front of their star, from the perspective of Earth. This technique is called transit photometry, but no measureable transit has been seen yet, and there's only a 1.5 percent possibility one occurring for Proxima b, said the researchers, because of Earth's poor angle in relation to Proxima Centauri.
There is another way to estimate radius, however, by simulating the behaviour of what is thought the planet is made from. This is the method used by a team of French and American researchers at the Marseille Astrophysics Laboratory and the Department of Astronomy at Cornell University.
The scientists calculated that the radius of the planet is between 0.94 and 1.4 times that of Earth. If it's towards the lower end of that calculation, with a radius of around 6,000 kilometers, the team at CNRS said that the planet would be very dense and contain a metallic core that accounts for two-thirds of the planet's mass. This core is would be surrounded by a rocky mantle.
At a maximum radius of 8,920 kilometers, the team said that Proxima b's mass would be evenly split between a rocky center and a surrounding ocean, some 200 kilometers deep.
"In both cases, a thin, gassy atmosphere could surround the planet, like on Earth, rendering Proxima b potentially habitable," the scientists concluded.
Get six of our favorite Motherboard stories every day by signing up for our newsletter.