A catalogue of 25 “hot Jupiter” exoplanets

An exoplanet is a planet outside of our Solar System. While the first possible exoplanet observation was recorded in 1917, the existence of exoplanets was not confirmed until 1992.

Using decades of data from two telescopes, the Hubble and the Spitzer, an international team of astronomers have begun to catalogue different types of exoplanets, identifying 25 “hot Jupiters” and publishing their findings in the Astrophysical Journal Supplement Series. A hot Jupiter is a class of gas giant exoplanet that is physically similar to Jupiter, but has very short orbital periods of less than 10 days – our Jupiter takes 12 years!

The Hubble space telescope, launched in 1990 and orbiting the Earth 15 times per day about 550km above the surface, provided 600 hours of observations. The Spitzer infrared telescope, which was active from 2003 to 2020, provided 400 hours of complementary observations, capturing low-energy light sources including cooler objects or those moving away from the observer. The data included eclipses for all 25 exoplanets, and transits (when a planet passes between a star and the observer) for 17 of them, providing crucial information about the exoplanets’ atmospheres.

“Our paper marks a turning point for the field: we are now moving from the characterisation of individual exoplanet atmospheres to the characterisation of atmospheric populations,” says Dr Billy Edwards, co-leader of the study from the University College London (UCL), UK.

One of the main findings was the characterisation of thermal inversions typically found in ultra-hot giant planet atmospheres. This is where the temperature of the atmosphere, instead of getting gradually cooler away from the planet, displays a pocket of warmer atmosphere, similar to what occurs on Earth’s ozone layer.

They found that the cooler planets in their population sample (less than 1,726°C) had noninverted thermal profiles with signatures of water absorption, while the hotter planet in the sample (greater than 1,726°C) had inverted thermal profiles. Almost all the inverted thermal profiles had signatures of titanium oxide (TiO), vanadium oxide (VO) and FeH (iron hydride) that are sufficiently stable in an atmosphere at such high temperatures.

“Many issues such as the origins of the water on Earth, the formation of the Moon, and the different evolutionary histories of Earth and Mars are still unsolved despite our ability to obtain in-situ measurements,” says Quentin Changeat, lead author and astrophysicist from UCL. “Large exoplanet population studies, such as the one we present here, aim at understanding those general processes.”