In about five billion years, our Sun will exit its main sequence phase and transition to its red giant phase. At this point, the Sun will expand and consume the planets of the inner Solar System, including Mercury and Venus. What will become of Earth when this happens has been the subject of debate for many decades. But with the recent explosion in exoplanet discoveries, 5,759 confirmed in 4,305 systems so far, astronomers hope to learn more about how planets fare as their stars near the end of their life cycle.
Using the 10-meter telescope at the Keck Observatory in Hawaii, an international team of astronomers discovered an Earth-like planet orbiting a white dwarf star 4,000 light-years from Earth. This planet orbits its star, about half the mass of our Sun, at a distance roughly twice that of the Earth today. The system resembles what is expected to become of our system once the Sun has exhausted the last of its fuel and blows off its outer layers in a supernova. It also offers some assurances that Earth will survive the Sun becoming a red giant and exploding in a supernova.
The team was led by 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. He was joined by multiple colleagues from UC Berkeley, UC San Diego, Tsinghua University, the Harvard & Smithsonian Center for Astrophysics (CfA), the California Institute of Technology (Caltech), the University of Washington, Ohio State University, the University of Maryland, and the NASA Goddard Space Flight Center. The paper that details their findings recently appeared in the journal Nature Astronomy.
To break it down, the Sun’s expansion as it becomes a red giant will likely mean the destruction of Mercury and Venus. At the same time, the Sun’s decreasing mass will force the surviving planets to migrate to more distant orbits, which could include Earth. If Earth survives when the Sun finally goes supernova, it will probably end up orbiting the resulting white dwarf remnant at a distance of 2 astronomical units (AUs) – twice its current distance. As Zhang related in a UC Berkeley News release,
“We do not currently have a consensus whether Earth could avoid being engulfed by the red giant sun in 6 billion years. 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.”
This is what astronomers may have found when they observed this planetary system roughly 4,000 light-years away. Located near the bulge at the center of our galaxy, this system was first noticed in 2020 when it passed in front of another star located 25,000 light-years from Earth. This caused a microlensing event, where the powerful gravity of the white dwarf focused and amplified the light of the background star by a factor of 1,000. The event was first detected by the Korea Microlensing Telescope Network (NMTNet) in the Southern Hemisphere, leading the team to designate it KMT-2020-BLG-0414.
The team estimated that the system included a star about half the mass of our Sun, an Earth-sized planet, and a likely brown dwarf with 17 times the mass of Jupiter. The analysis also concluded that the Earth-sized planet orbited its star at a distance of between 1 and 2 AUs. At the time, it was difficult to identify the type of star because neighboring stars and the magnified background star obscured its light. By 2023, the lensing event had passed, which made it possible for the team to examine the lensing system more closely using the Keck II 10-meter telescope in Hawaii.
As Zhang indicated, the team took two separate images but detected nothing. Since the lensing star was dark and low mass, they concluded it could only be a white dwarf. As noted, scientists are unsure what will happen to Earth when it reaches its red giant phase or if it will survive to orbit the white star remnant. This planetary system provides an example of a planet that did survive its sun expanding and exploding in a supernova. However, there is little chance of it being habitable since it orbits beyond the white dwarf’s habitable zone.
What’s more, some research suggests that if the expanding Sun doesn’t engulf our planet, it will eventually blow our atmosphere off and vaporize Earth’s oceans. Said co-author Jessica Lu, an associate professor and chair of astronomy at UC Berkeley:
“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. 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.”
In addition, Zhang and his colleagues resolved an ambiguity regarding the location of the brown dwarf. According to the original analysis, the brown dwarf had a very wide Neptune-like or Mercury-type orbit. In the latter case, this would make it a hot brown dwarf, similar to the many “Hot Jupiters” observed repeatedly beyond our Solar System. Zhang and his colleagues could rule the latter scenario since a closely-orbited brown dwarf would have been consumed once the star entered its red giant phase.
This ambiguity resulted from “microlensing degeneracy,” where two distinct lensing configurations can give rise to the same lensing effect. Luckily, Zhang and co-author Bloom discovered a similar degeneracy in 2022 using a machine-learning algorithm designed to analyze microlensing simulations. When they applied the same technique to KMT-2020-BLG-0414, they were able to rule out alternative models of the planetary system. As Bloom explained:
“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. 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.”
This system offers many opportunities for follow-up observations by next-generation telescopes like the Nancy Grace Roman Space Telescope (RST), scheduled for launch in 2027. One of the main objectives of the RST is to measure light curves from microlensing events to find 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,” said Bloom.
The findings would seem to confirm another theory about the fate of our Solar System. When the Sun expands, our system’s habitable zone will migrate to the outer Solar System. If humanity is still around at this time, it will need to migrate to the icy satellites that orbit Jupiter and Saturn, which are likely to become planets covered in deep oceans – giving new meaning to the words “Ocean Worlds.”
Further Reading: Berkeley News, Nature Astronomy
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