The very small motion of a small star has revealed the presence of a super-Earth exoplanet, orbiting at a distance close to habitable.
Around a faint red dwarf named Ross 508, located just 36.5 light-years away (still too dark to be seen with the naked eye), astronomers have confirmed the existence of a world only 4 times the mass of the Earth. Given what we know about the limits of planetary mass, this means that the world is likely to be terrestrial, or rocky, rather than gaseous.
The exoplanet, called Ross 508 b, is unlikely to be habitable for life as we know it; however, the discovery, the first in a new survey using the Japan National Astronomical Observatory (NAOJ) Subaru Telescope in Hawaii, demonstrates the effectiveness of the techniques used to locate small planets around faint stars.
The search for habitable exoplanets is somewhat hampered by the very nature of how we believe these exoplanets are. The only template we have is Earth: a relatively small planet, orbiting at a distance from its star where temperatures favor liquid water on the surface. This is known as the “living area”.
These are not the only factors at stake, obviously: Mars is within the habitable zone of the Sun, for example, but they are the easiest to detect.
However, the techniques we use to search for exoplanets work best in large worlds, such as gas giants, which orbit at very close distances, too hot for liquid water. That’s not to say we can’t find other types of worlds, but it’s harder.
The main technique for finding exoplanets is the transit method. This is what NASA, TESS, and Kepler exoplanet hunting telescopes used before. An instrument looks at the stars, looking for regular drops in its light, caused by an object that regularly orbits between us and the star.
The depth of this traffic can be used to calculate the mass of the object; the larger the light curve, caused by larger planets, the easier it is to detect.
At the time of writing, 3,858 exoplanets have been confirmed with this method.
The second most fruitful technique is the radial velocity method, also known as the oscillation method or Doppler. When two bodies are locked in orbit, one does not orbit the other; rather, they orbit a center of mutual gravity. This means that the gravitational influence of any orbiting planet causes a star to sway slightly in place, yes, even the Sun.
Thus, the star of the light of the stars that reaches the Earth has a very weak Doppler displacement. As it moves toward us, light is compressed slightly into bluer wavelengths, and as it moves away, it stretches into redder wavelengths. This technique is best for detecting smaller exoplanets with wider orbits.
In 2019, an international team of astronomers led by NAOJ embarked on a survey using the Subaru Telescope to search for exoplanets in faint red dwarf stars by identifying Doppler changes in near-infrared and near-infrared wavelengths. This allows you to look for weaker red dwarf stars, and therefore older and established ones.
Ross 508 b, described in an article directed by astronomer Hiroki Harakawa of the Subaru Telescope, is the first exoplanet in the campaign, and is promising. The world has about 4 times the mass of the Sun, orbiting the star every 10.75 days.
This is much closer than Earth’s orbit, you may have noticed; but Ross 508 is much smaller and fainter than the Sun. At this distance, the stellar radiation reaching Ross 508 b is only 1.4 times the solar radiation reaching the Earth. This places the exoplanet very close to the outer outer edge of its star’s habitable zone.
The discovery is very good for the future. On the one hand, Ross 508 b travels through its star. This means that TESS, which turned to the celestial sector of the star in April and May of this year, may have obtained enough traffic data for astronomers to discern whether the exoplanet has an atmosphere. These observations can help scientists characterize world atmospheres that may be more habitable.
In addition, Ross 508, with 18 percent of the mass of the Sun, is one of the smallest and faintest stars with a world in orbit discovered by radial velocity. This suggests that future radial velocity surveys at infrared wavelengths have the potential to detect a large number of exoplanets orbiting faint stars and reveal the diversity of their planetary systems.
The team’s research has been accepted into the publications of the Astronomical Society of Japan and is available on arXiv.