A microlensing event observed in 2020 resulted from a planetary system that includes a planet similar to Earth and a brown dwarf. Initially, the type of star was uncertain, but researchers have confirmed it to be a white dwarf. This star system mirrors the predicted future of our own sun-Earth arrangement in about 8 billion years. The positive takeaway is that the planet withstood its star’s red giant phase, indicating Earth might endure a similar fate. Unfortunately, it remains uninhabitable.
A newfound Earth-like planet located 4,000 light years away in the Milky Way gives us insight into a potential future for our own planet billions of years from now, after our sun has transformed into a white dwarf and a desolate, frozen Earth orbits far beyond Mars.
The distant planetary system was discovered by astronomers at the University of California, Berkeley, using the Keck 10-meter telescope in Hawaii. It closely resembles what we expect from our sun-Earth system: a white dwarf around half the mass of the sun and an Earth-sized planet in an orbit that is double the current distance of Earth’s orbit.
This scenario likely reflects Earth’s future. The sun will eventually swell into a red giant, extending beyond Earth’s current orbit and consuming Mercury and Venus. During this expansion, as the sun loses mass, planets may shift to farther orbits, potentially giving Earth a chance to survive further from the sun. Eventually, as the outer layers of the red giant dissipate, a white dwarf will remain—a small, dense remnant with a mass comparable to a star. If Earth makes it through this transformation, it will probably end up in an orbit twice as far from the sun as it is now.
This discovery, set for publication in the journal Nature Astronomy, offers insights into how main sequence stars, such as the sun, evolve through the red giant stage into white dwarfs and the implications for surrounding planets. Some research indicates that the sun may start this shift in about 1 billion years, potentially vaporizing Earth’s oceans and doubling its orbital radius—assuming it doesn’t engulf our planet first.
In roughly 8 billion years, the sun’s outer layers will have dispersed, leaving a dense, luminous white dwarf that is about half the mass of the sun, yet smaller than Earth.
“Currently, there’s no agreement on whether Earth can dodge being engulfed by the sun when it enters its red giant phase in 6 billion years,” stated Keming Zhang, the study’s lead, a former PhD student at UC Berkeley and a postdoctoral fellow at UC San Diego. “What we do know is that Earth will likely be uninhabitable in about a billion years due to a drastic greenhouse effect that will boil away the oceans—long before the sun becomes a red giant.”
This unique planetary system illustrates one case where a planet survived, but it exists well outside the habitable zone of the dim white dwarf and is not expected to host any life. It may have had conditions suitable for life at an earlier time when its star was more like our sun.
“Whether life can persist on Earth throughout this red giant phase is uncertain. Still, the most critical issue is that Earth doesn’t get devoured by the sun as it grows into a red giant,” said Jessica Lu, an associate professor and chair of astronomy at UC Berkeley. “The planetary system that Keming discovered showcases a planet, likely an Earth-like one originally in an orbit similar to ours, that made it through the red giant phase of its host star.”
Microlensing phenomenon magnifies starlight dramatically
This distant planetary system, located near the galactic bulge, caught astronomers’ attention in 2020 when it passed in front of a more distant star, magnifying that star’s light by a factor of 1,000. The gravity from the system acted as a lens to enhance the light from the background star.
The team that observed this phenomenon nicknamed it KMT-2020-BLG-0414, because the Korea Microlensing Telescope Network in the Southern Hemisphere detected it. Although the brightening of the background star—also within the Milky Way but about 25,000 light years away—was merely a pinprick of light, its brightness fluctuations over two months enabled researchers to conclude that the system comprises a star about half the mass of the sun, an Earth-mass planet, and a massive planet approximately 17 times the mass of Jupiter—a likely brown dwarf. These brown dwarfs are essentially failed stars, lingering just below the threshold needed to ignite nuclear fusion in their cores.
The analysis also indicated that the Earth-like planet orbits between 1 and 2 astronomical units from the star—roughly twice the distance separating Earth from the sun. However, the type of star hosting the planet remained unclear due to the light being obscured by the brightness of the magnified background star and a few nearby stars.
To identify the star’s nature, Zhang and his team, including UC Berkeley astronomers Jessica Lu and Joshua Bloom, conducted a follow-up study in 2023 with the Keck II 10-meter telescope, equipped with adaptive optics to reduce atmospheric blur. With the background star that was once magnified 1,000 times now dimmed, they expected to see the lensing star if it were a typical main-sequence star like our sun, according to Lu.
However, Zhang found no sign of it in two separate images taken with Keck.
“Our conclusions stem from ruling out other scenarios since a normal star would have been easily identifiable,” Zhang explained. “Because the lensing star was both dark and low mass, we concluded that it must be a white dwarf.”
“Interestingly, this instance exemplifies how observing nothing can be more intriguing than observing something,” Lu remarked, as she searches for microlensing events caused by wandering stellar-mass black holes in the Milky Way.
Detecting exoplanets via microlensing
This discovery is part of Zhang’s project to investigate microlensing events that reveal the presence of planets, aiming to better understand the types of stars that exoplanets orbit.
<p”There is some luck involved, as fewer than one in ten microlensing events featuring planets are expected to involve white dwarfs,” Zhang stated.
The new observations also clarified the position of the brown dwarf within the system. “Initially, we couldn’t determine if the brown dwarf orbited in a very wide path akin to Neptune’s or well within Mercury’s orbit. However, since we now confirm it orbits a stellar remnant, the latter scenario seems improbable, as such a giant planet would more likely have been consumed,” Zhang explained, referring to a category of planets known as hot Jupiters that tend to have very small orbits.
The ambiguity in modeling arises from microlensing degeneracy, where different lensing configurations can lead to the same observed effect. This was a concept that Zhang and Bloom explored in 2022 through AI analysis of microlensing simulations. He also employed the same AI techniques to exclude alternative models for KMT-2020-BLG-0414 that could have been overlooked.
“Microlensing has evolved into a fascinating means of exploring other stellar systems that cannot be detected through conventional methods, such as transit or radial velocity methods,” Bloom noted. “There’s a plethora of worlds that we’re beginning to unlock through microlensing, making it an exciting time as we approach the discovery of unusual configurations like this one.”
A primary aim of NASA’s upcoming Nancy Grace Roman Telescope, set to launch in 2027, is to capture light curves from microlensing events to find exoplanets, many of which will require additional observations using other telescopes to identify the stars that harbor these exoplanets.
“Careful follow-up is essential with leading-edge facilities, such as adaptive optics and the Keck Observatory. This needs to be ongoing—not just a day or a month later, but numerous years into the future—after the lens has separated from the background star, allowing us to resolve the observations,” Bloom advised.
Zhang reflected that even if Earth gets engulfed during the sun’s red giant expansion in a billion years, humans might find refuge in the outer solar system. Moons such as Europa, Callisto, and Ganymede around Jupiter, as well as Enceladus near Saturn, appear to hold frozen water oceans that could eventually thaw as the outer layers of the expanding red giant unfold.
“As the sun swells into a red giant, the habitable zone will shift toward the orbits of Jupiter and Saturn, transforming many of these moons into potential ocean worlds,” Zhang suggested. “In such a scenario, humanity could migrate to those locations.”
Other co-authors include Weicheng Zang and Shude Mao from Tsinghua University in Beijing, China, who contributed to the initial 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 from the University of Washington in Seattle; B. Scott Gaudi of The Ohio State University in Columbus; Quinn Konopacky from UC San Diego; Natalie LeBaron from UC Berkeley; and Sean Terry from the University of Maryland in College Park.
