The radial velocity signal from the exoplanet candidate orbiting the star HD 26965 (40 Eridani A) — host to Mr. Spock’s fictional home planet, Vulcan, in the Star Trek Universe — is stellar activity, according to new research.
HD 26965 is a bright K-class dwarf star located 16 light-years away in the constellation of Eridanus.
Also known as 40 Eridani A, the star is a member of the triple star system 40 Eridani.
It is approximately 6.9 billion years old, has a mass about 78% of the Sun’s and a radius 87% as large.
In 2018, astronomers announced the possible detection of a super-Earth exoplanet in a 42-day orbit around HD 26965.
But the scientists cautioned that it could turn out to be messy stellar jitters masquerading as a planet.
“Two methods for detecting exoplanets dominate all others in the continuing search for strange new worlds,” said Dartmouth College astronomer Abigail Burrows and colleagues.
“The transit method, watching for the tiny dip in starlight as a planet crosses the face of its star, is responsible for the vast majority of detections.”
“But the radial velocity method also has racked up a healthy share of exoplanet discoveries. This method is especially important for systems with planets that don’t, from Earth’s point of view, cross the faces of their stars.”
“By tracking subtle shifts in starlight, scientists can measure wobbles in the star itself, as the gravity of an orbiting planet tugs it one way, then another.”
“For very large planets, the radial velocity signal mostly leads to unambiguous planet detections. But not-so-large planets can be problematic.”
In a new study, the astronomers analyzed data from NEID, a radial velocity instrument mounted on the 3.5-m WIYN Telescope at Kitt Peak National Observatory.
“NEID, like other radial velocity instruments, relies on the Doppler effect: shifts in the light spectrum of a star that reveal its wobbling motions,” they explained.
“In this case, parsing out the supposed planet signal at various wavelengths of light, emitted from different levels of the star’s outer shell, or photosphere, revealed significant differences between individual wavelength measurements — their Doppler shifts — and the total signal when they were all combined.”
“That means, in all likelihood, the planet signal is really the flickering of something on the star’s surface that coincides with a 42-day rotation — perhaps the roiling of hotter and cooler layers beneath the star’s surface, called convection, combined with stellar surface features such as spots and plages, which are bright, active regions. Both can alter a star’s radial velocity signals.”
“While the new finding, at least for now, robs the star 40 Eridani A of its possible planet Vulcan, the news isn’t all bad.”
“The demonstration of such finely tuned radial velocity measurements holds out the promise of making sharper observational distinctions between actual planets and the shakes and rattles on surfaces of distant stars.”
The study was published in the Astronomical Journal.
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Abigail Burrows et al. 2024. NEID Reveals that the Planet Candidate Orbiting HD 26965 is Stellar Activity. AJ 167, 243; doi: 10.3847/1538-3881/ad34d5
