Maunakea, Hawaiʻi – The discovery of an Earth-like planet 4,000 light years away in the Milky Way galaxy provides a preview of one possible fate for our planet billions of years in the future, when the Sun has turned into a white dwarf, and a blasted and frozen Earth has migrated beyond the orbit of Mars.
This distant planetary system, identified by a University of California (UC) Berkeley-led team of astronomers after observations with the W. M. Keck Observatory on Maunakea, Hawaiʻi Island, looks very similar to expectations for the Sun-Earth system: it consists of a white dwarf about half the mass of the Sun and an Earth-size companion in an orbit twice as large as Earth’s today.
That is likely to be Earth’s fate. The Sun will eventually inflate like a balloon larger than Earth’s orbit today, engulfing Mercury and Venus in the process. As the star expands to become a red giant, its decreasing mass will force planets to migrate to more distant orbits, offering Earth a slim opportunity to survive farther from the Sun. Eventually, the outer layers of the red giant will be blown away to leave 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 probably end up in an orbit twice its current size.
The discovery, published online today 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.
“We do not currently have a consensus whether Earth could avoid being engulfed by the red giant Sun in 6 billion years,” said study lead author Keming Zhang, 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 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 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.”
MICROLENSING MAKES STAR BRIGHTEN A THOUSANDFOLD
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 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
To identify the type of the host star, Zhang, Lu, and fellow UC Berkeley astronomer Joshua Bloom looked more closely at the lensing system in 2023 using Keck Observatory’s second generation Near-Infrared Camera (NIRC2) paired with the Observatory’s adaptive optics system to eliminate the blur caused by Earth’s atmosphere.
But Zhang detected nothing in two separate Keck Observatory 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.
“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.”
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.”
Learn more:
- “This rocky planet around a white dwarf resembles Earth — 8 billion years from now” – UC Berkeley Press Release, September 26
TOP PHOTO: Artist’s illustration of a distant white dwarf with an Earth-like planet in an orbit just beyond where Mars is in our solar system. Earth could end up in such an orbit circling a white dwarf in about 8 billion years, if, like this exoplanet, it can survive the Sun’s red giant phase on its way to becoming a white dwarf. Credit: W. M. Keck Observatory/Adam Makarenko
ABOUT NIRC2
The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
ABOUT ADAPTIVE OPTICS
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and current systems now deliver images three to four times sharper than the Hubble Space Telescope at near-infrared wavelengths. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.