Our closest stellar neighbor is Proxima Centauri, an M-type (red dwarf) star located over 4.24 light-years away (part of the Alpha Centauri trinary system). In 2016, the astronomical community was astounded to learn that an Earth-like planet orbited within this star’s circumsolar habitable zone (HZ). In addition to being the closest exoplanet to Earth, Proxima b was also considered the most promising place to look for extraterrestrial life for a time.

Unfortunately, the scientific community has been divided on whether or not life could even be possible on this planet. All of these studies indicate that this question cannot be answered until astronomers characterize Proxima b’s atmosphere, ideally by observing it as it passes in front (aka. transited) of its host star. But in a new NASA-supported study, a team led by astrophysicists at the University of Chicago determined that this is an unlikely possibility.

The study that describes their findings, which will appear soon in Frontiers in Astronomy and Space Sciences, was led by Emily A. Gilbert, a Graduate Student with the University of Chicago’s Department of Astronomy and Astrophysics. She was joined by researchers from The Adler Planetarium, the Center for Space Science and Technology (University of Maryland), and the Exoplanets and Stellar Astrophysics Laboratory at the NASA Goddard Space Flight Center.

TOI 1338 b is a circumbinary planet orbiting its two stars. It was discovered by TESS. Credit: NASA’s Goddard Space Flight Center/Chris Smith

The people responsible for the discovery were Guillem Anglada-Escudé and a team of astronomers from the Pale Red Dot campaign. Using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the ESO 3.6-metre telescope at the ESO’s La Silla Observatory, the team confirmed the presence of Proxima b using a method known as Doppler Spectroscopy (aka. the Radial Velocity Method).

This method consists of observing the spectra of stars for signs of “wobble,” where the star is moving closer and farther away from Earth. This is caused by the gravitational influence of planets that orbit the star, the extent of which is used to infer the mass of the planets. In the case of Proxima b, astronomers obtained a minimum mass estimate of 1.24 and a maximum estimate of 2.06 Earth masses.

Its presence was confirmed again in 2020 using the ESO’s Very Large Telescope (VLT) and its Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) instrument – the successors to the HARPS spectrograph. While most exoplanets to date have been detected using the Transit Method (aka. Transit Photometry), this was deemed impractical for a star like Proxima Centauri, which is a low-mass and less bright M-type (red dwarf) star.

But as Gilbert and her colleagues indicate in their study, this has not prevented numerous astronomical research teams from trying to detect planets transitting in front of Proxima Centauri. For example. Prof. Kipping and his colleagues from the Cool Worlds Laboratory at Columbia University observed Proxima Centauri for 43.5 days between 2014-15 using the Canadian Space Agency’s Microvariability and Oscillation of Stars (MOST) satellite.

An artist’s rendition of the Transiting Exoplanet Survey Satellite (TESS). Credit: NASA’s Goddard Space Flight Center

In 2016, two research teams independently observed Proxima Centauri for
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