8 Billion Years Ahead: Earth’s Fate Revealed by a Distant Twin Planet

Astronomers have discovered an Earth-like planet orbiting a white dwarf 4,000 light years away, offering a glimpse into our own planet’s distant future.

The discovery suggests that Earth might escape being swallowed by the sun when it expands into a red giant, moving to a larger orbit and potentially ending up around a white dwarf.

Discovery of an Earth-like Planet

The discovery of an Earth-like planet 4,000 light years away in the Milky Way offers a glimpse into what our own planet’s fate could be billions of years from now. By then, the sun will have transformed into a white dwarf, and Earth may be a frozen, barren world orbiting far beyond its current path.

This distant planetary system, discovered by astronomers at the University of California, Berkeley, using the Keck 10-meter telescope in Hawaii, bears a striking resemblance to what scientists predict for the future sun-Earth system. It consists of a white dwarf, roughly half the mass of our sun, and an Earth-sized planet orbiting at twice the Earth-Sun distance.

Earth’s Inevitable Cosmic Evolution

This scenario could indeed be Earth’s destiny. As the sun ages, it will expand into a red giant, swelling beyond Earth’s current orbit and engulfing Mercury and Venus. As it loses mass, its gravitational hold will weaken, causing the remaining planets, including Earth, to drift to more distant orbits. Eventually, the red giant will shed its outer layers, leaving only a dense white dwarf core, the size of a planet with the mass of a star. If Earth survives this process, it might find itself orbiting this stellar remnant at about twice its current distance from the sun.

The discovery, to be published this week in the journal Nature Astronomy, tells scientists about the evolution of main sequence stars, like the sun, through the red giant phase to a white dwarf, and how it affects the planets around them. Some studies suggest that for the sun, this process could begin in about 1 billion years, eventually vaporizing Earth’s oceans and doubling Earth’s orbital radius — if the expanding star doesn’t engulf our planet first.

Eventually, about 8 billion years from now, the sun’s outer layers will have dispersed to leave behind a dense, glowing ball — a white dwarf — that is about half the mass of the sun, but smaller in size than Earth.

A video depicting one possible fate for Earth when the Sun expands into a red giant. If the red giant sheds its mass quickly enough to allow Earth to migrate to a wider orbit, it will escape being engulfed by the expanding surface of the red giant, eventually settling into an orbit about twice its current size. In the process, however, it will be heated to a lava planet, becoming uninhabitable long before the red giant becomes a white dwarf. Scientists have found one example of an Earth-like planet that escaped destruction and now orbits a white dwarf, showing that it is possible. Credit: W. M. Keck Observatory/Adam Makarenko

Future Projections and Current Analyses

“We do not currently have a consensus whether Earth could avoid being engulfed by the red giant sun in 6 billion years,” said study leader Keming Zhang, a former doctoral student at the University of California, 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 around 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.”

The planetary system provides one example of a planet that did survive, though it is far outside the habitable zone of the dim white dwarf and unlikely to harbor life. It may have had habitable conditions at some point, when its host was still a sun-like star.

“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. “This system that Keming’s found is an 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.”

Insights From Microlensing Events

The far-away planetary system, located near the bulge at the center of our galaxy, came to astronomers’ attention in 2020 when it passed in front of a more distant star and magnified that star’s light by a factor of 1,000. The gravity of the system acted like a lens to focus and amplify the light from the background star.

The team that discovered this “microlensing event” dubbed it KMT-2020-BLG-0414 because it was detected by the Korea Microlensing Telescope Network in the Southern Hemisphere. The magnification of the background star — also in the Milky Way, but about 25,000 light years from Earth — was still only a pinprick of light. Nevertheless, its variation in intensity over about two months allowed the team to estimate that the system included a star about half the mass of the sun, a planet about the mass of Earth and a very large planet about 17 times the mass of Jupiter — likely a brown dwarf. Brown dwarfs are failed stars, with a mass just shy of that required to ignite fusion in the core.

The analysis also concluded that the Earth-like planet was between 1 and 2 astronomical units from the star — that is, about twice the distance between the Earth and sun. It was unclear what kind of star the host was because its light was lost in the glare of the magnified background star and a few nearby stars.

Expanding the Boundaries of Exoplanet Research

To identify the type of star, Zhang and his colleagues, including UC Berkeley astronomers Jessica Lu and Joshua Bloom, looked more closely at the lensing system in 2023 using the Keck II 10-meter telescope in Hawaii, which is outfitted with adaptive optics to eliminate blur from the atmosphere. Because they observed the system three years after the lensing event, the background star that had once been magnified 1,000 times had become faint enough that the lensing star should have been visible if it was a typical main-sequence star like the sun, Lu said.

But Zhang detected nothing in two separate Keck images.

“Our conclusions are based on ruling out the alternative scenarios, since a normal star would have been easily seen,” Zhang said. “Because the lens is both dark and low mass, we concluded that it can only be a white dwarf.”

“This is a case of where seeing nothing is actually more interesting than seeing something,” said Lu, who looks for microlensing events caused by free-floating stellar-mass black holes in the Milky Way.

Finding Exoplanets Through Microlensing

The discovery is part of a project by Zhang to more closely study microlensing events that show the presence of a planet, in order to understand what types of stars exoplanets live around.

“There is some luck involved, because you’d expect fewer than one in 10 microlensing stars with planets to be white dwarfs,” Zhang said.

The new observations also allowed Zhang and colleagues to resolve an ambiguity regarding the location of the brown dwarf.

“The original analysis showed that the brown dwarf is either in a very wide orbit, like Neptune’s, or well within Mercury’s orbit. Giant planets on very small orbits are actually quite common outside the solar system,” Zhang said, referring to a class of planets called hot Jupiters. “But since we now know it is orbiting a stellar remnant, this is unlikely, as it would have been engulfed.”

The modeling ambiguity is caused by so-called microlensing degeneracy, where two distinct lensing configurations can give rise to the same lensing effect. This degeneracy is related to the one Zhang and Bloom discovered in 2022 using an AI method to analyze microlensing simulations. Zhang also applied the same AI technique to rule out alternative models for KMT-2020-BLG-0414 that may have been missed.

“Microlensing has turned into a very interesting way of studying other star systems that can’t be observed and detected by the conventional means, i.e. the transit method or the radial velocity method,” Bloom said. “There is a whole set of worlds that are now opening up to us through the microlensing channel, and what’s exciting is that we’re on the precipice of finding exotic configurations like this.”

Implications for Humanity’s Future

One purpose of NASA’s Nancy Grace Roman Telescope, scheduled for launch in 2027, is to measure light curves from microlensing events to find exoplanets, many of which will need follow up using other telescopes to identify the types of stars hosting the exoplanets.

“What is required is careful follow up with the world’s best facilities, i.e. adaptive optics and the Keck Observatory, not just a day or a month later, but many, many years into the future, after the lens has moved away from the background star so you can start disambiguating what you’re seeing,” Bloom said.

Zhang noted that even if Earth gets engulfed during the sun’s red giant phase in a billion or so years, humanity may find a refuge in the outer solar system. Several moons of Jupiter, such as Europa, Callisto and Ganymede, and Enceladus around Saturn, appear to have frozen water oceans that will likely thaw as the outer layers of the red giant expand.

“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,” Zhang said. “I think, in that case, humanity could migrate out there.”

Reference: “An Earth-mass planet and a brown dwarf in orbit around a white dwarf” by Keming Zhang, Weicheng Zang, Kareem El-Badry, Jessica R. Lu, Joshua S. Bloom, Eric Agol, B. Scott Gaudi, Quinn Konopacky, Natalie LeBaron, Shude Mao and Sean Terry, 26 September 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02375-9

Other co-authors are Weicheng Zang and Shude Mao of Tsinghua University in Beijing, China, who co-authored the first paper about KMT-2020-BLG-0414; former UC Berkeley doctoral student Kareem El-Badry, now an assistant professor at the California Institute of Technology in Pasadena; Eric Agol of the University of Washington in Seattle; B. Scott Gaudi of The Ohio State University in Columbus; Quinn Konopacky of UC San Diego; Natalie LeBaron of UC Berkeley; and Sean Terry of the University of Maryland in College Park.

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