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In 2018, astronomers detected an exoplanet around the star 40 Eridani. It\u2019s about 16 light-years away in the constellation Eridanus. The discovery generated a wave of interest for a couple of reasons. Not only is it the closest Super-Earth around a star similar to our Sun, but the star system is the fictional home of Star Trek\u2019s Vulcan science officer, Mr. Spock. <\/p>\n
It\u2019s always fun when a real science discovery lines up with science fiction.<\/p>\n
<\/p>\n Eridani\u2019s other name is HD 26965, and it\u2019s actually a triple-star system. Astronomers discovered the system\u2019s lone planet, Eridani b, using the radial velocity method. Orbiting planets tug on their stars, and the star\u2019s movement creates a change in its spectrum. Astronomical telescopes with spectrometers can detect the changes. <\/p>\n Jian Ge, an astronomy professor at the University of Florida, led the study that presented the discovery in 2018. At the time, Ge said in a press release, \u201cThe new planet is a \u2018super-Earth\u2019 orbiting the star HD 26965, which is only 16 light years from Earth, making it the closest super-Earth orbiting another Sun-like star. The planet is roughly twice the size of Earth and orbits its star with a 42-day period just inside the star\u2019s optimal habitable zone.\u201d<\/p>\n A super-Earth in the habitable zone around a Sun-similar star \u2018only\u2019 16 light-years away is an intriguing discovery. Its link with a beloved Star Trek character gave the discovery wings, and word spread. <\/p>\n However, in the intervening years, follow-up observations have not confirmed Eridani b\u2019s existence. A 2021 study suggested that the change in the star\u2019s spectrum was a false positive. Now, a new study says that the exoplanet fondly named Vulcan does not exist. <\/p>\n The study is \u201cThe Death of Vulcan: NEID Reveals That the Planet Candidate Orbiting HD 26965 Is Stellar Activity.\u201d It\u2019s published in The Astronomical Journal, and the lead author is Abigail Burrows, an astronomer at Dartmouth College.<\/p>\n \u201cWe revisit the long-studied radial velocity (RV) target HD 26965 using recent observations from the NASA-NSF \u201cNEID\u201d precision Doppler facility,\u201d Burrows and her co-authors write. After a deeper, line-by-line analysis of the radial velocity data, \u201c\u2026 we demonstrate that the claimed 45-day signal previously identified as a planet candidate is most likely an activity-induced signal.\u201d<\/p>\n Activity-induced signal means that the signal comes from the star\u2019s activity, not from the external tug of an exoplanet. <\/p>\n Vulcan\u2019s initial detection was based on data from the Dharma Planet Survey (DPS.) DPS monitored about 150 nearby Sun-like stars for changes in their spectra. Data from the Keck Telescope and the HARPS planet-finding spectrograph also contributed to the discovery. <\/p>\n When the planet was detected in 2018, the discoverers recommended caution. They presented the data as they collected it, along with their best interpretation. That\u2019s standard in science, and they were careful in calling it a candidate planet. In their paper, they also discussed \u201cthe possibility that the RV signal is actually produced by stellar rotation modulated activity.\u201d That activity could be sunspots, convection irregularities, or other things. <\/p>\n But in the end, they concluded that what they were seeing was likely a planet. <\/p>\n \u201cBy carefully examining the RV data in the active and quiet phases of the star, and after carefully considering all possible stellar activity sources, we concluded that the coherent signal seen from HD 26965 is most likely from a planet, with some RV noise contributed by stellar activity,\u201d the authors wrote in the 2018 paper. <\/p>\n The rest of us were happy to agree because finding a super-Earth around a nearby Sun-like star is the kind of thing we hope to find.<\/p>\n \u201cMen sometimes see exactly what they wish to see.\u201d<\/p>\n -Spock of Vulcan<\/cite><\/p><\/blockquote>\n<\/figure>\n Sadly for Vulcan, the newest research shows that the stellar activity isn\u2019t noise. It accounts for the entire signal. <\/p>\n The new results are based on NEID, the NN-explore Exoplanet Investigations with Doppler spectroscopy. It\u2019s a high-resolution spectrometer attached to the WIYN (Wisconsin-Indiana-Yale-NOIRLab) telescope at Kitt Peak Observatory. The researchers used NEID to capture 63 spectra from Eridani over a six-month period. <\/p>\n NEID revealed a lot of information about the star, including things like contrast and radial velocity. Together, NEID data paints a more complete picture of the star and its activity. In this new work, Burrows and her co-researchers showed that all of this activity lines up with the star\u2019s 42-day rotation period. <\/p>\n \u201cAll measurements show a strong signal at or near the 42-day stellar rotation period,\u201d they write.<\/p>\n The authors write that their work \u201cpoints toward a decaying starspot or plage\u201d as the source of the signal. A plage is a bright spot on a star\u2019s chromosphere. They used a variety of methods to reach this conclusion. \u201cWhile each of these methods taken individually may not rule out a potential planetary signal at the same phase and period as the activity signal, collectively, our analyses show that an activity hypothesis is favoured over the specific planet claimed in Ma et al. (2018),\u201d they conclude.<\/p>\n \u201cWhen you eliminate the impossible, whatever remains, however improbable, must be the truth.\u201d <\/p>\n Spock of Vulcan<\/cite><\/p><\/blockquote>\n<\/figure>\n The authors of the new paper didn\u2019t set out to debunk Vulcan. Their paper is part of an effort to better understand the periodic and quasi-periodic spectral changes from Sun-like stars. Without a better understanding, annoying false positives will cloud our understanding of exoplanets, especially Earth-like ones around Sun-like stars. \u201cTo reach the precision necessary to detect temperate, Earth-mass extrasolar planets (exoplanets) around Sun-like stars using the radial velocity (RV) technique, the community must improve Doppler measurement precision significantly from the current state of the art,\u201d they write.<\/p>\n \u201cDetecting and characterizing these exo-Earths is vital for future spaceborne direct imaging missions, which will set the scientific priorities for the coming decade,\u201d the authors explain. <\/p>\n\n
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