A team of astronomers has uncovered an Earth-like planet orbiting a star located 4,000 light years away from the solar system, potentially offering insights into Earth’s distant future. The rocky planet, about the same mass as Earth, revolves around a white dwarf in the constellation Sagittarius.

The discovery brings a glimmer of hope for Earth’s survival when our sun enters its final stages. It suggests that Earth could potentially avoid being consumed by the expanding sun, opening up possibilities for human migration to the outer solar system, with moons such as Europa, Callisto, and Ganymede around Jupiter, or Enceladus near Saturn, becoming possible havens for future generations.

What is a white dwarf?

A white dwarf is a star’s remnant after it has run out of nuclear fuel and shed its outer layers. It symbolises the sun’s eventual fate. The sun will grow into a red giant as its nuclear fuel runs out, then shrink to become a white dwarf. The extent of its expansion will determine which planets in the solar system will be engulfed – Mercury and Venus are likely to be consumed. But what about Earth?

In a study published in Nature Astronomy, researchers from the University of California, Berkeley, used the Keck Telescope in Hawaii to observe a system designated KMT-2020-BLG-0414. The system contains a white dwarf star with an Earth-sized planet in an orbit twice as far from the star as Earth is from the sun. Alongside the planet is a brown dwarf – a planet roughly 17 times the mass of Jupiter.

This finding supports the theory that as the sun expands into a red giant, its loss of mass will push the planets into more distant orbits. This phenomenon could allow Earth to escape destruction. Jessica Lu, an associate professor of astronomy at UC Berkeley, noted, “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.”

Future of Earth

“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 the runaway greenhouse effect-long before the risk of getting swallowed by the red giant.”  

Could humanity find refuge beyond Earth? As the sun swells into a red giant, the habitable zone in the solar system will shift outward to the orbits of Jupiter and Saturn. Many of their moons, such as Europa and Callisto, could become ocean worlds capable of supporting life. Zhang suggested, “Humanity could migrate out there.”

An artist’s impression shows a rocky planet orbiting a stellar remnant called a white dwarf roughly 4,200 light years from Earth in this undated illustration.
| Photo Credit: Reuters

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 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 (September 26, 2024) in the journal Nature Astronomy.

The sun, roughly four and a half 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,” Mr. 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,” Mr. 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, Mr. 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,” Mr. 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,” Mr. Zhang added.

“There’s hope,” Mr. Zhang said.