Existence of Earth-like planet orbiting dead sun offers hope for our planet’s ultimate survival

The discovery of an Earth-like planet 4,000 light-years away in the Milky Way galaxy offers a glimpse of our planet’s possible fate billions of years from now, when the Sun has transformed into a white dwarf and a frozen, destroyed Earth has migrated beyond the orbit of Mars.

This distant planetary system, identified by astronomers at the University of California, Berkeley, after observations with the 10-meter Keck telescope in Hawaii, closely resembles expectations for a Sun-Earth system: It consists of a white dwarf about half the mass of the Sun and an Earth-sized companion in an orbit twice as large as Earth’s today.

This will likely be Earth’s fate. The Sun will eventually swell like a balloon larger than Earth’s current orbit, swallowing Mercury and Venus in the process. As the star expands to become a red giant, its diminishing mass will force the planets to migrate to more distant orbits, giving Earth a slim chance of surviving further from the Sun. Eventually, the outer layers of the red giant will be blown away, leaving behind a dense white dwarf no larger than a planet but with the mass of a star. If Earth has survived by then, it will likely end up in an orbit twice its current size.

The discovery, to be published in the journal Astronomy of naturetells scientists how main-sequence stars, like the Sun, evolve from red giant to white dwarf, and how this 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 first engulf our planet.

Eventually, in about 8 billion years, the Sun’s outer layers will have scattered away, leaving behind a dense, bright ball – a white dwarf – that will be about half the mass of the Sun but smaller than Earth.

“We don’t currently have a consensus on whether Earth could avoid being swallowed 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 the Eric and Wendy Schmidt AI in Science postdoctoral fellow at UC San Diego.

“In any case, planet Earth will only be habitable for about another billion years, after which the Earth’s oceans will be vaporized by the runaway greenhouse effect – well before the risk of being swallowed up by the red giant.”

The planetary system provides an example of a planet that has survived, although it is located well outside the habitable zone of the white dwarf and is unlikely to support life. It may have experienced habitable conditions at one time, when its host was still a Sun-like star.

“It’s not known whether life can survive on Earth during this (red giant) period. But the most important thing is certainly that Earth doesn’t get swallowed by the Sun when it becomes a red giant,” said Jessica Lu, associate professor and chair of the astronomy department at the University of California, Berkeley. “This system that Keming discovered is an example of a planet – probably an Earth-like planet, initially in an Earth-like orbit – that survived the red giant phase of its host star.”

Microlensing multiplies the brightness of stars by a thousand

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

The team that discovered this “microlensing event” named it KMT-2020-BLG-0414 because it was detected by the Korean 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 just a tiny point of light.

Still, the intensity variation over about two months allowed the team to estimate that the system included a star with a mass about half that of the Sun, a planet with a mass close to that of Earth, and a very large planet with a mass about 17 times that of Jupiter, probably a brown dwarf. Brown dwarfs are failed stars, with masses just below that required to trigger fusion in the core.

The analysis also concluded that the Earth-like planet was located one to two astronomical units from the star, about twice the distance between Earth and the Sun. It was unclear exactly what type of star it was, because its light was lost in the glare of the magnified background star and a few nearby stars.

To identify the type of star, Zhang and his colleagues, including astronomers Jessica Lu and Joshua Bloom of the University of California, Berkeley, took a closer look at the lensing system in 2023 using the 10-meter Keck II telescope in Hawaii, which is equipped with adaptive optics to remove atmospheric blur. As they observed the system three years after the lensing event, the background star that had been magnified 1,000 times had become faint enough that the lensing star would have been visible if it were a typical main-sequence star like the Sun, Lu said.

But Zhang detected nothing on two separate Keck images.

“Our conclusions are based on eliminating alternative scenarios, because a normal star would have been easily visible,” 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 where seeing nothing is actually more interesting than seeing something,” said Lu, who is looking for microlensing events caused by stellar-mass black holes floating freely in the Milky Way.

Discovery of exoplanets thanks to microlensing

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

“There is an element of luck in this, because you would expect that less than one in ten microlensing stars with planets would be a white dwarf,” Zhang said.

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

“The initial analysis showed that the brown dwarf is either in a very large orbit, like Neptune’s, or well inside the orbit of Mercury. Giant planets in 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 that it orbits a stellar remnant, that’s unlikely, because it would have been swallowed up.”

The modeling ambiguity is caused by something called microlensing degeneracy, where two distinct lens configurations can give rise to the same lensing effect. This degeneracy is related to one that 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 might have been missed.

“Microlensing has become a very interesting method for studying other stellar systems that cannot be observed and detected by conventional means, such as the transit method or the radial velocity method,” Bloom said. “There are a whole set of worlds that are now opening up to us through the microlensing channel, and the exciting thing is that we are on the verge of discovering exotic configurations like this one.”

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

“What’s needed is careful follow-up with the best facilities in the world, which is adaptive optics and the Keck Observatory, not just a day or a month later, but many years into the future, after the lens has moved away from the background star so that you can begin to disambiguate what you’re seeing,” Bloom said.

Zhang noted that even if Earth were swallowed up during the Sun’s red giant phase in about a billion years, humanity could find refuge in the outer solar system. Several of Jupiter’s moons, such as Europa, Callisto and Ganymede, and Saturn’s Enceladus, appear to have frozen water oceans that will likely melt as the red giant’s outer layers expand.

“As the Sun becomes a red giant, the habitable zone will move around the orbit of Jupiter and Saturn, and many of these moons will become ocean planets,” Zhang said. “I think, in that case, humanity could migrate to these regions.”

Other co-authors include Weicheng Zang and Shude Mao of Tsinghua University in Beijing, China, who co-authored the first paper on 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 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.