Astronomers using the Keck II 10-m telescope have discovered evidence for an Earth-mass planet and a brown dwarf orbiting a white dwarf star. The planet has a mass of 1.9 Earth masses and orbits the white dwarf at a distance of 2.1 AU (astronomical units) — about twice the distance between the Earth and Sun.
An artist’s impression of an Earth-like planet in an orbit around a white dwarf. Image credit: Adam Makarenko.
The newly-discovered planetary system, located around 4,000 light-years away from Earth, 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.
Astronomers that discovered this so-called microlensing event dubbed it KMT-2020-BLG-0414 because it was detected by the Korea Microlensing Telescope Network.
The magnification of the background star — also in the Milky Way, but about 25,000 light-years from Earth — was still only a pinprick of light.
Nevertheless, its variation in intensity over about two months allowed the team to estimate that the system included a star about half the mass of the Sun, a 1.9-Earth-mass planet and a brown dwarf about 17 times the mass of Jupiter.
The analysis also concluded that the Earth-like planet was approximately 2.1 AU from the star.
It was unclear what kind of star the host 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, the astronomers looked more closely at the lensing system in 2023 using the Keck II 10-m telescope in Hawaii, which is outfitted with adaptive optics to eliminate blur from the atmosphere.
“Because they observed the system three years after the lensing event, the background star that had once been magnified 1,000 times had become faint enough that the lensing star should have been visible if it was a typical main-sequence star like the Sun,” said University of California, Berkeley astronomer Jessica Lu.
Earth could end up in 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.
“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,” said Dr. Keming Zhang, a postdoctoral researcher at the University of Californi, San Diego.
“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.”
“I think, in that case, humanity could migrate out there.”
The findings were published in the journal Nature Astronomy.
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K. Zhang et al. An Earth-mass planet and a brown dwarf in orbit around a white dwarf. Nat Astron, published online September 26, 2024; doi: 10.1038/s41550-024-02375-9