The discovery of a rocky exoplanet 4,000 light-years away in the Milky Way offers a possible glimpse into the fate of Earth billions of years from now, when the Sun will collapse into a white dwarf and a frozen Earth will drift beyond the orbit of Mars.

This distant planetary system was identified by astronomers at the University of California, Berkeley, using observations made with the 10-meter Keck Telescope in Hawaii.

The fate of the Sun-Earth system

The planetary system reflects what scientists predict will be the ultimate fate of the Sun-Earth system: a white dwarf about half the size of the Sun and an Earth-sized moon orbiting at twice the distance the Earth orbits the Sun today. This could also be the fate of the Earth.

The Sun is expected to expand beyond Earth’s current orbit and become a red giant, swallowing Mercury and Venus in the process. As the Sun loses mass, its gravitational pull weakens, causing the planets to migrate outward. The Earth can avoid complete destruction by moving away from the Sun.

Eventually the Sun will shed its outer layers, leaving behind a dense white dwarf no larger than Earth but about the mass of a star. If the Earth survives this process, it can settle into an orbit twice as far from the Sun.

Evolution of main sequence stars

The new discovery helps scientists better understand how main sequence stars like the Sun become red giants and eventually white dwarfs, and how these changes affect the planets around them.

Some research suggests that for the Sun, this process could begin in about a billion years, causing Earth’s oceans to evaporate and the radius of Earth’s orbit to double – assuming the Earth is not initially engulfed by the expanding sun. Eventually, after about eight billion years, the Sun will shed its outer layers, leaving behind a white dwarf with half the mass of the Sun but smaller than Earth.

What we know about the fate of the world

“Currently, we do not have a consensus on whether Earth will recover from being engulfed by a red giant sun within six billion years,” said study leader Keming Zhang, a former postdoctoral researcher at UC Berkeley and now a research assistant professor. Postdoctoral researcher at UC San Diego.

“In any case, planet Earth will be habitable for another billion years or so, after which Earth’s oceans will evaporate due to the greenhouse effect, long before they risk being swallowed by a red giant.”

A story of survival

The discovered planetary system is an example of a planet that has survived despite being well outside the habitable zone of a faint white dwarf and unlikely to support life. Perhaps it was habitable while its star was still sunlike.

“It is unknown whether life can survive on Earth during this (red giant) period. But of course the most important thing is that the Earth will not be 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 found by Keming is an example of an Earth-like planet that survived the red giant phase of its host star, possibly in an Earth-like orbit.”

Characteristics of a distant planetary system

A distant planet system near the bulge at the center of the Milky Way caught astronomers’ attention in 2020 when it passed in front of a more distant star, amplifying the star’s light by 1,000 times. The system’s gravity acted as a lens, focusing and magnifying the light of the background star. The discovery, called KMT-2020-BLG-0414, was made by the Korean Southern Hemisphere Microlensing Telescope Network.

The background star, located about 25,000 light-years from Earth, appeared as a small point of light. The change in its brightness over two months allowed scientists to characterize the planetary system.

Experts estimate that the system contains a star about half the size of the Sun, an Earth-sized planet, and a much larger planet (possibly a brown dwarf, a type of failed star) about 17 times the size of Jupiter. They have enough mass to ignite fusion.

The mysterious personality of the leading actor

The analysis shows that the Earth-sized planet orbits at a distance of one to two astronomical units from the star, about twice the distance between the Earth and the Sun. But the type of star hosting the planet remained unclear because the planet’s light was blocked by a magnified background star.

To determine the nature of the star, Zhang and colleagues, including UC Berkeley astronomers Jessica Lu and Joshua Bloom, studied the system in 2023 with the 10-meter Keck II telescope in Hawaii, which uses adaptive optics to correct for atmospheric blurring.

When they observed the system three years after the microlensing event, the light of the background star had become so dim that the lensed star could be seen as if it were a typical main sequence star like the Sun. However, Zhang could not find a visible star in two separate Keck images.

Stars orbiting outer planets

“Since a normal star would be easy to see, our conclusions are based on eliminating alternative scenarios,” Zhang said. “Since the lens is dark and low-mass, we concluded that it could only be a white dwarf.”

“This is a case where not seeing anything is actually more interesting than seeing something,” added Lu, who is also looking for microlensing events caused by free-floating stellar-mass black holes in the Milky Way.

The discovery is part of Zhang’s project to study microlensing events that reveal the presence of planets, with the aim of learning more about the types of stars orbiting these exoplanets.

“There is also some chance involved because you would expect less than one in 10 microlens stars with planets to be a white dwarf,” Zhang explained.

New worlds open with micro lensing

“Microlensing has become a very interesting way to study other star systems that cannot be observed and detected by traditional methods such as the transit method or the radial velocity method,” Bloom said.

“There are a number of worlds now opening up to us through the microlensing channel, and the exciting thing is that we are on the verge of finding exotic configurations like this.”

One of the main goals of NASA’s Nancy Grace Telescope in Rome, scheduled to launch in 2027, is to capture light curves from microlensing events to detect exoplanets. Many will need further observations with other telescopes to identify the types of stars that host planets.

“It takes close observation with the best instruments in the world, adaptive optics and the Keck Observatory, not just a day or a month later, but many, many years after the lens has moved away from the background star, so you can begin to disambiguate what you’re seeing,” Bloom added.

Humanity’s fate beyond Earth

Zhang noted that even if the Earth is swallowed by the Sun in the red giant stage, humanity can take shelter in the outer solar system. Some of the moons of Jupiter and Saturn, such as Europa, Callisto, Ganymede, and Enceladus, contain oceans of frozen water that may melt as the sun expands.

“Once the Sun becomes a red giant, the habitable zone will shift around the orbits of Jupiter and Saturn, and many of these moons will become oceanic planets. “In this case, I think humanity can migrate there,” Zhang concluded. The study was published in the journal Nature Astronomy.