The study of extrasolar planets has revealed some interesting things in recent decades. Not only have astronomers discovered entirely new types of planets – Super Jupiters, Hot Jupiters, Super-Earths, Mini-Neptunes, etc. – it has also revealed new things about solar system architecture and planetary dynamics. For example, astronomers have seen multiple systems of planets where the orbits of the planets did not conform to our Solar System.

According to a new study led by the University of Bern, an international team of researchers recently observed a Mini-Neptune (TOI-2257 b) orbiting a red dwarf star r\oughly 188.5 light-years from Earth. What was interesting about this find was how the small ice giant had such an eccentric orbit, which is almost twice as long as it is wide! This is almost two and a half times as eccentric as Mercury, making TOI-2257 b the most eccentric planet ever discovered!

The study was the work of the SAINT-EX consortium, made up of researchers from the Center for Space and Habitability (CSH) at the University of Bern and the National Center of Competence in Research (NCCR) PlanetS (jointly-run by the University of Bern and Geneva). They were joined by members from the ESA’s European Space Research and Technology Centre (ESTEC), NASA’s Jet Propulsion Laboratory (JPL), the NASA Ames Research Center, and multiple universities and research institutes.

An artist’s rendition of the Transiting Exoplanet Survey Satellite (TESS). Credit: NASA’s Goddard Space Flight Center

For the sake of their study, the team relied on data obtained by the Transiting Exoplanet Survey Satellite (TESS), which observed the red dwarf star TOI-2257 for four months and noticed repeated dips in luminosity. This is known as the Transit Method (Transit Photometry), where periodic dips in brightness are considered to be possible indications of a planet passing in front of its face (aka. transiting) relative to the observer.

However, the gaps between observations during those four months created a measure of uncertainty. While the dips indicated the presence of an exoplanet measuring 2.2 Earth radii, it was unclear if the planet had an orbital period of 176, 88, 59, 44, or 35 days. Hence, the team combined the TESS data with observations from the Search And characterIsatioN of Transiting EXoplanets (SAINT-X), the TRAPPIST-North, and the Fred Lawrence Whipple Observatory (FLWO) telescopes.

The SAINT-EX telescope, in particular, helped confirm the planet’s orbital period after witnessing partial transit after 35 days, followed by many more transits with a period of 35 days. Another 35 days later, SAINT-EX was able to observe the entire transit, which gave us even more information about the properties of the system,” said Robert Wells from the CSH, a co-author on the study involved in the data processing.

This 35-day period indicates that TOI-2257 b orbits within its parent star’s circumsolar habitable zone (HZ), the distance where liquid water can exist on its surface. The shorter orbital period also makes it easier to study this planet since scientists will be able to observe transits regularly, thereby increasing opportunities to measure light from the parent star passing through its atmosphere. This produces spectra, which astronomers can use to determine the chemical composition of the planet’s atmosphere.

An artist’s rendering of five planets orbiting TOI-1233, four of which were discovered using the Transiting Exoplanet Satellite Survey