What happens at the end of our world? Scientists have found an Earth-like planet orbiting a star 4,000 light years away from the solar system that may reveal our planet's ultimate fate.
The rocky planet of similar mass to Earth orbits a white dwarf star -- the dense remains of a star that's exhausted its nuclear fuel and sheds its outer layers -- in the constellation Sagittarius.
Its existence means that Earth may escape the fiery clutches of an expanding sun -- and make it possible for humans to flee to the outer solar system, possibly to moons like Europa, Callisto and Ganymede at Jupiter and Enceladus around Saturn.
A white dwarf is the inevitable end-state of our sun, but before it reaches that state, it will go through a much more violent process. As it begins to run out of fuel, it will become a red giant star, expanding into the solar system before shrinking to become a white dwarf.
How far the red giant expands determines which planets will be engulfed and destroyed -- with Mercury and Venus likely to be consumed. What about Earth?
In a paper published in Nature Astronomy, a team from the University of California, Berkeley, used the Keck Telescope in Hawaii to study a system called KMT-2020-BLG-0414, finding a white dwarf with an Earth-size planet in an orbit twice as large as Earth's of the sun. There is also a brown dwarf -- a huge planet about 17 times the mass of Jupiter -- in orbit.
This example of a planet -- probably an Earth-like planet originally on a similar orbit to Earth -- that survived its host star's red giant phase gives credence to the theory that as the sun expands to become a red giant, its decreasing mass will force planets to migrate to more distant orbits' thereby saving Earth from being destroyed.
"Whether life can survive on Earth through that (red giant) period is unknown. But certainly, the most important thing is that Earth isn't swallowed by the Sun when it becomes a red giant," said Jessica Lu, associate professor and chair of astronomy at UC Berkeley.
Earth may technically survive the sun's red giant phase, but its future is not rosy. Scientists think the sun could begin bulging in as little as a billion years or as many as six billion years. Either way, despite doubling the size of Earth's orbital path, the sun's expansion will eventually vaporize Earth's oceans and leave it as a hot lava planet and completely uninhabitable. By eight billion years, what's left of Earth could orbit a white dwarf.
"We do not currently have a consensus whether Earth could avoid being engulfed by the red giant sun in six billion years," said Keming Zhang, the lead author and a former doctoral student at the UC Berkeley, who is now an Eric and Wendy Schmidt AI in Science Postdoctoral fellow at UC San Diego. "In any case, planet Earth will only be habitable for another billion years, at which point Earth's oceans would be vaporized by runaway greenhouse effect -- long before the risk of getting swallowed by the red giant."
Could humanity find a refuge in the outer solar system? "As the sun becomes a red giant, the habitable zone will move to around Jupiter and Saturn's orbit, and many of these moons will become ocean planets," said Zhang. "Humanity could migrate out there."