Astronomers Look at Super-Earths That had Their Atmospheres Stripped Away by Their Stars

As the planets of our Solar System demonstrate, understanding the solar dynamics of a system is a crucial aspect of determining habitability. Because of its protective magnetic field, Earth has maintained a fluffy atmosphere for billions of years, ensuring a stable climate for life to evolve. In contrast, other rocky planets that orbit our Sun are either airless, have super-dense (Venus), or have very thin atmospheres (Mars) due to their interactions with the Sun.

In recent years, astronomers have been on the lookout for this same process when studying extrasolar planets. For instance, an international team of astronomers led by the National Astronomical Observatory of Japan (NAOJ) recently conducted follow-up observations of two Super-Earths that orbit very closely to their respective stars. These planets, which have no thick primordial atmospheres, represent a chance to investigate the evolution of atmospheres on hot rocky planets.

The study that described their findings, which were recently published in The Astrophysical Journal, was led by Dr. Teruyuki Hirano of the NAOJ and The Graduate University for Advanced Studies (SOKENDAI) in Tokyo, Japan. He was joined by researchers from the Instituto de Astrofísica de Canarias (IAC), the SETI Institute at NASA’s Ames Research Center, the Harvard-Smithson Center for Astrophysics (CfA), the University of Tokyo, and many other institutes.

Artist’s impression of Super-Earths TOI-1634b and TOI-1685b. Credit: NASA Exoplanet Catalog

Dr. Hirano and his team chose two planets originally identified NASA’s Transitting Exoplanet Survey Spacecraft (TESS) – TOI-1634b and TOI-1685b. These two Super-Earth planets that orbit M-type (red dwarf) stars located about 114 and 122 light-years away (respectively) in the constellation Perseus. Using the InfraRed Doppler (IRD) spectrograph mounted on the 8.5 m (~28 ft) Subaru Telescope, the team made multiple confirmations about these two rocky exoplanets.

For starters, Dr. Hirano and his colleagues confirmed that the candidates are rocky super-Earths that measure 1.7 and 1.79 Earth radii and are 4.91 and 3.78 times as massive. They also confirmed that they have ultra-short orbital periods, taking 24 and less than 17 hours to complete a single orbit around their stars. This makes TOI-1634b one of the largest and most massive ultra-short-period rocky exoplanets confirmed to date.

But most importantly, the spectra they obtained provided insight into these planets’ internal and atmospheric structures. What they found was that they were “bare,” meaning that they lacked a primordial hydrogen-helium atmosphere, similar to what Earth had billions of years ago. In all likelihood, this is a result of the planets’ proximity with their host stars, which are prone to flare activity.

In addition, the “bare” nature of these rocky planets raises the possibility of a secondary atmosphere caused by volcanic outgassing. This is also what took place on Earth billions ca. 2.5 billion years ago, which led to Earth transitioning from a hydrogen-helium atmosphere to one composed predominantly of carbon dioxide, sulfur dioxide, and other gases that originated inside our planet.

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser

Therefore, these planets are a major opportunity for studying how atmospheres evolve on rocky planets, especially ones that orbit red dwarf stars. In addition, the fact that these planets are “bare” means that astronomers will be able to test theories regarding rocky planets that orbit closely to red dwarf stars. Compared to G-type yellow dwarfs (like the Sun), red dwarfs are known for being variable and prone to flare-ups.

Since rocky planets that orbit within a red dwarf’s habitable zone are likely to be tidally locked (with one side constantly facing towards the star), astronomers are naturally curious if they can maintain their atmospheres for long. Red dwarfs make up an estimated 75% of stars in the Milky Way, and many rocky planets have been found in red dwarf systems (including Proxima b, which orbits the closest star to our own).

For all of these reasons, studying these exoplanets could have significant implications in the search for extraterrestrial life. At the same time, it will help astronomers learn more about how this particular class of planet (Super-Earths) form and evolve. “Our project to intensively follow-up planetary candidates identified by TESS with the Subaru Telescope is still in progress, and many unusual planets will be confirmed in the next few years,” said Dr. Hirano.

In the near future, further observations will be possible using next-generation telescopes, including the James Webb Space Telescope (JWST). Along with several ground-based observatories, astronomers will have the necessary instruments to detect and characterize the atmospheres of these planets.

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