Cool exoplanet reveals missing link between hot Jupiters and cold solar system planets

Research into a rare planet is revealing the link between hot Jupiter-sized exoplanets and cold solar system giants like Saturn.

Astronomers searching for exoplanets (planets outside of our solar system) have investigated a Saturn-sized body around a Sun-like star, 490 lightyears from Earth.

The research, led by The University of Warwick in collaboration with other global institutions, focused on a transiting exoplanet called TOI-2447 b, which is much cooler and further away from its host star than most known exoplanets.

This study is key to understanding exoplanets which have properties between those of the known exoplanet population (mostly Jupiter-sized planets that are close to their host star and hence very hot) and our solar-system giants (which are much further away from the Sun and very cold).

While TOI-2447 b is a similar size to Saturn, it orbits its host star every 69 days, which is much slower than most known exoplanets but much faster than the 29 years it takes Saturn to orbit the Sun. It is far enough out from its star to be much cooler than most exoplanets found to date, just 140 degrees Celsius (like a cool oven) compared to searing temperatures of more than a thousand degrees, but still much warmer than the -180 degrees of Saturn. These intermediate properties make TOI-2447 b particularly intriguing and a useful case for testing theories of planet formation.

During their investigations, the team also found signs of at least one more giant exoplanet with a much longer orbit - much further out and cooler than TOI-2447 b. These giant planets are not conducive to life as we know it, for the same reasons Saturn isn’t - the temperature, pressure and chemical makeup are too extreme. However, according to the researchers, it is likely that smaller planets also exist in this same system - including the chance of a rocky outer planet that is conducive to life.

Research Fellow Dr Samuel Gill, from The University of Warwick, who led the study, said: "These planets bridge the gap between the numerous well-studied planets around other stars that we have found to reside on orbits shorter than that of Mercury and planets more akin to the gas and ice giants in our own solar system".

Dr Solène Ulmer-Moll, from the University of Geneva and University of Bern, who was also involved in the study, said: "Such planets provide useful test cases for models of planet formation and evolution. Without them, our understanding of exoplanets is weighted towards short-period, large, and hot exoplanets that are unlike our own solar system".

The exoplanet was originally identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, which found it to ’transit’ across its host star. Transits occur when bodies, such as planets, pass on the line of sight between the Earth and the star, partially occulting the star - causing a decrease in its brightness that can be observed and quantified. These dips in brightness can reveal both the presence of the planet and its size.

Unlike many planets identified using TESS, which tend to be close to their host star and hot, TOI-2447 b was shown to transit only once - meaning it has a long orbit and is far away from its host star. There are very few well-studied planets that are this far away from their host and have temperatures below 200 degrees Celsius.

In this study, astronomers proved that the transit observed with TESS was due to a planet, and they measured its size and weight. They used the telescopes of the Next Generation Transit Survey (NGTS), run by the University of Warwick, to show that it takes TOI-2447 b 69 days to orbit its host star.

Sam Gill added: "This planet joins a growing number of long-period planets identified by TESS and then solved with the NGTS. NGTS is one of the few observatories with the stability, precision, and observational capability to follow up investigations on this exoplanet.

"Next we will investigate the evolution of this exoplanet and how it got to its current position in its star system. We will also try to probe the chemical composition of its warm atmosphere with the James Webb Space Telescope (JWST)."­s/2024arXi­v240507367­G/abstract