Astronomers have made an exciting discovery by spotting four new “mini-Neptune” planets around a distant star. These exoplanets are categorized as mini-Neptunes, a class of planets smaller and located farther from their stars compared to hot Jupiters.
The newly found planets revolve around different stars and have a size similar to that of Neptune or slightly smaller, ranging from two to five times the size of Earth. Two of these exoplanets orbit the same orange dwarf star called HD 15906, situated approximately 149 light-years away from our solar system.
A team led by researchers from the University of Bern and the University of Geneva in Switzerland identified the mini-Neptune using the Characterising Exoplanet Satellite (CHEOPS), the European Space Agency’s spacecraft dedicated to exoplanet exploration. NASA’s Transiting Exoplanet Survey Satellite (TESS) initially alerted CHEOPS to the presence of these exoplanets. Both instruments detect the dimming of light caused by exoplanets as they transit or cross the face of their parent stars.
Hugh Osborn, an astrophysicist at the University of Bern involved in the research, praised TESS for its ability to detect exoplanet transits, even for challenging small planets. However, TESS changes its field of view every 27 days to survey a large portion of the sky quickly, which limits its ability to detect planets with longer orbital periods.
TESS observed a single transit for two of the exoplanets, namely TOI 5678 b and HIP 9618 c. These planets, approximately 4.9 and 3.4 times the size of Earth, respectively, passed in front of their parent stars. However, these observations alone were insufficient to confirm the existence of the planets.
CHEOPS played a vital role in confirming these exoplanets by adopting a targeted approach based on the hints provided by TESS to spot additional transits, efficiently utilizing observation time. They pointed CHEOPS towards a specific target at a given time and, based on the presence or absence of a transit, eliminated possibilities and repeated the process until a unique orbital period solution was obtained.
After four or five attempts with software that proposed and prioritized candidate periods for each planet, the team successfully confirmed the existence of the two exoplanets. They determined that TOI 5678 b orbits its star every 48 Earth days, while HIP 9618 c orbits its star, located around 230 light-years away from our solar system, every 53 Earth days.
Using the same technique, the astronomers also identified two other mini-Neptunes, HD 15906 b and c, in a different planetary system. HD 15906 b has a diameter approximately 2.3 times that of Earth and orbits its orange dwarf star every 11 Earth days, while HD 15906 c has a diameter just under three times that of Earth and appears to orbit the same star at a greater distance, completing a circle every 22 Earth days.
To obtain a rough estimate of the planets’ masses, the astronomers used ground-based telescopes to observe the “wobbles” caused by these worlds in the motion of their parent stars. From this data, they calculated the densities of the exoplanets. However, this information alone was insufficient to determine the compositions of the mini-Neptunes.
The mini-Neptune location farther from their parent stars compared to typical hot Jupiters, as well as their relatively cooler temperatures, suggests the presence of atmospheres and clouds that the JWST could investigate. Moreover, the smaller size and longer orbital periods of these mini-Neptunes make them ideal candidates for studying Earth-sized planets around other stars.