The very small movement of a small star revealed the presence of a super-Earth exoplanet, orbiting at a distance close to the habitable.
Around a pale red dwarf called Ross 508, located just 36.5 light-years away (but too dim to see with the naked eye), astronomers have confirmed the existence of a world just 4 times the mass of the Earth. Given what we know of planetary mass limits, this means the world is likely to be terrestrial, or rocky, rather than gaseous.
The exoplanet, named Ross 508b, is unlikely to be habitable for life as we know it; however, the discovery, a first for a new study using the Subaru Telescope at the National Astronomical Observatory of Japan (NAOJ) in Hawaii, demonstrates the effectiveness of techniques used to locate small planets around dim stars.
The hunt for habitable exoplanets is somewhat hampered by the very nature of what we believe to be these exoplanets. The only template we have is the Earth: a relatively small planet, orbiting at a distance from its star, where the temperatures are conducive to liquid water on the surface. This is called the “habitable zone”.
These are obviously not the only factors at play – Mars is in the Sun’s habitable zone, for example – but they are the easiest to detect.
However, the techniques we use to search for exoplanets work best on large worlds, like gas giants, orbiting at very close distances, too hot for liquid water. That doesn’t mean 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’s exoplanet-hunting telescope, TESS, and Kepler before it use. An instrument stares at stars, looking for regular dips in their light, caused by an object that regularly orbits between us and the star.
The depth of this transit can be used to calculate the mass of the object; the larger the light curve – caused by larger planets – the easier it is to spot.
At the time of writing, 3,858 exoplanets discovered using this method have been confirmed.
The second most successful technique is the radial velocity method, also known as the oscillation method or the Doppler method. When two bodies are stuck in orbit, one does not orbit the other; rather they orbit around a mutual center of gravity. This means that the gravitational influence of any orbiting planet causes a star to wobble slightly in place – yes, even the Sun.
Thus, the starlight star reaching Earth is very slightly Doppler shifted. As it travels towards us, the light is slightly compressed into bluer wavelengths, and as it travels away, it is stretched into redder wavelengths. This technique is more effective at detecting smaller exoplanets with larger orbits.
In 2019, an international team of astronomers led by NAOJ embarked on a survey using the Subaru Telescope to search for dark red dwarf stars in search of exoplanets by identifying Doppler shifts in wavelengths infrared and near infrared. This allows a search for fainter, and therefore older and more established, red dwarf stars.
Ross 508 b, described in a paper led by Subaru Telescope astronomer Hiroki Harakawa, is the campaign’s first exoplanet, and it shows promise. The world is about 4 times the mass of the Sun, orbiting the star every 10.75 days.
It’s 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 striking Ross 508 b is only 1.4 times the solar radiation striking Earth. This places the exoplanet very close to the outer inner edge of its star’s habitable zone.
The discovery bodes very well for the future. On the one hand, Ross 508 b transits its star. This means that TESS, which was shot at the star’s sky sector in April and May this year, may have obtained enough transit data for astronomers to discern whether the exoplanet has an atmosphere. Such observations can help scientists characterize the atmospheres of worlds that might be more habitable.
Additionally, Ross 508, at 18% the mass of the Sun, is one of the smallest and faintest stars with an orbiting world discovered using radial velocity. This suggests that future radial velocity studies in infrared wavelengths have the potential to uncover a vast treasure trove of exoplanets orbiting dim stars and reveal the diversity of their planetary systems.
The team’s research has been accepted into the Publications of the Astronomical Society of Japanand is available on arXiv.