The very small motion of a small star has revealed the presence of a superterrestrial exoplanet, orbiting at a distance nearly habitable.
Around a faint red dwarf called Ross 508, just 36.5 light-years away (but too dim to see with the naked eye), astronomers have confirmed the existence of a world only 4 times the mass of Earth. Given what we know about planetary mass limits, that means the world is probably terrestrial or rocky rather than gaseous.
The exoplanet, called Ross 508 b, is probably not 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 minor planets around faint stars.
The hunt for habitable exoplanets is somewhat hampered by the nature of what we think those exoplanets look like. The only template we have is Earth: a relatively small planet that orbits at a distance from its star and where the temperature is favorable for liquid water on the surface. This is the so-called ‘habitable zone’.
However, the techniques we use to search for exoplanets work best on large worlds, such as gas giants, which orbit very short distances and are too hot for liquid water. That doesn’t mean we can’t find other types of worlds, but it’s more difficult.
The most important technique for finding exoplanets is the transit method. This is what NASA’s exoplanet-hunting telescope TESS uses, and Kepler before that. An instrument stares at stars, looking for regular dips in their light caused by an object that orbits regularly between us and the star.
The depth of this transit can be used to calculate the mass of the object; the wider the light curve – caused by larger planets – the easier it is to see.
At the moment of writing, 3,858 exoplanets found using this method have been confirmed.
The second most fruitful technique is the radial velocity method, also known as the wobble or Doppler method. When two bodies are locked in orbit, one does not revolve around the other; instead, they revolve around a mutual center of gravity. This means that the gravitational influence of planets orbiting the Earth causes a star to wobble a bit — yes, even the sun†
So the starlight that reaches Earth is very weakly Doppler-shifted. When it moves toward us, the light is slightly compressed into bluer wavelengths, and when it moves away from us, it is stretched into redder wavelengths. This technique is better at detecting smaller exoplanets with wider orbits.
In 2019, an international team of astronomers led by NAOJ began a study using the Subaru telescope to search faint red dwarf stars for exoplanets by identifying Doppler shifts in infrared and near infrared wavelengths. This leads to a search for fainter, and thus 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’s a promising one. The world is about 4 times the mass of the sun and orbits 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 that distance, the stellar radiation hitting Ross 508 b is only 1.4 times the solar radiation hitting Earth. This places the exoplanet very close to the outer inner edge of its star’s habitable zone.
The discovery bodes well for the future. First, Ross 508 b passes through his star. This means that TESS, which was pointed at the star sector in the sky in April and May this year, may have obtained enough transit data for astronomers to discern whether the exoplanet has an atmosphere. Such observations could help scientists characterize atmospheres of worlds that may be more habitable.
In addition, Ross 508, at 18 percent the mass of the Sun, is one of the smallest and faintest stars with a spinning world discovered using radial velocity. This suggests that future radial velocity studies in infrared wavelengths have the potential to reveal a vast array of exoplanets orbiting faint stars and reveal the diversity of their planetary systems.
The team’s research has been accepted in the Publications of the Astronomical Society of Japanand is available on arXiv†
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