Frigid alien planet may offer a glimpse at Earth's distant future

The first rocky planet ever spotted orbiting a burned out star called a white dwarf offers a glimpse of what may be in store for Earth billions of years from now - showing it is possible our planet might survive the death of the sun, albeit as a cold and desolate outpost in space.

The planet, with a mass about 1.9 times that of Earth, is orbiting the white dwarf about 4,200 light-years away from our solar system near the bulge at the center of the Milky Way galaxy, according to a study using data from Hawaii-based telescopes. A light year is the distance light travels in a year, about 5.9 trillion miles (9.5 trillion km).

The white dwarf began as an ordinary star one or two times the mass of the sun. Its current mass is about half of the sun’s. Stars with a mass less than eight times the sun's end their lives as a white dwarf, the most common type of stellar remnant.

Before its host star's death, the planet orbited at a distance possibly placing it in the "habitable zone" - not too hot and not too cold, where liquid water could exist on the surface and perhaps support life. It originally orbited at about the same distance as Earth is to the sun. Following its star's demise, it is at 2.1 times that distance.

"It's currently a freezing world because the white dwarf, which is in fact smaller than the planet, is extremely faint compared to when it was a normal star," said University of California, San Diego astronomer Keming Zhang, lead author of the study published on Thursday in the journal Nature Astronomy, opens new tab.

The sun, roughly 4-1/2 billion years old, is destined to become a white dwarf.

"At the end of our sun's life, it will puff up to enormous size - astronomers call it a red giant - and gently blow off its outer layers in a wind," University of California, Berkeley astronomer and study co-author Jessica Lu said.

"As our sun loses mass, the planets' orbits will expand to larger sizes. Eventually, the sun loses all of its outer layers and leaves behind a hot compact core. This is called a white dwarf."

Astronomers have debated whether Earth - the third planet from the sun, with Venus the second and Mars the fourth - would be engulfed and destroyed when the sun expands during its red giant phase, estimated to occur seven billion years from now. It will become a white dwarf a billion years after that.

"Theoretical models disagree as to whether Earth could survive. Venus will most certainly be engulfed whereas Mars will most certainly survive. Our modeling shows that this planet very likely had a similar orbit to Earth before its host star became a red giant. It implies that Earth's chances for survival may be higher than currently thought," Zhang said.

Until now, only gas-giant planets larger than Jupiter, our solar system's biggest planet, had been spotted orbiting white dwarfs.

The white dwarf is orbited by two bodies - the Earth-like planet and, further out, a brown dwarf, an object bigger than a planet but smaller than a star. The planet endured tough times during its star's death throes.

"It may have been a lava planet when the star became a red giant, then eventually cooled down to its current freezing state," Zhang said.

As the sun ages and heats up, our solar system's habitable zone would move outward. Earth will remain habitable for less than about a billion more years from now, by which point its oceans likely will have evaporated, Zhang said.

Does this mean certain doom for humankind - or whatever life still resides on Earth? "We must migrate out of Earth prior to the one-billion-year time scale," Zhang said.

By the time the sun becomes a red giant, certain large moons in the outer solar system like Jupiter's Ganymede and Saturn's Titan and Enceladus may offer a refuge, Zhang added. "There's hope," Zhang said.