Exoplanets that go to extremes

European astronomers are taking a close look at one of the hottest known exoplanets. Initial measurements made by the CHaracterising ExOPlanet Satellite (CHEOPS) space telescope indicate that the giant WASP-189b, located 326 light years from Earth in the constellation Libra, is an impressive 3200 degrees Celsius.

The exoplanet glows as hot as a small star as it orbits its central star at high speed on an unusual orbit that takes it close to the star’s poles.

“Planets like WASP-189b are called ultra-hot Jupiters,” says Monika Lendl, from Switzerland’s University of Geneva. “Iron melts at such a high temperature and even becomes gaseous. This object is one of the most extreme exoplanets we know so far.”

Lendl is lead author of a paper in the journal Astronomy & Astrophysics – the first publication using data from the telescope since it was placed in a sun-synchronous orbit 700 kilometres above Earth on 18 December last year.

CHEOPS is the first European Space Agency (ESA) mission dedicated to characterising known exoplanets (planets outside our Solar System). It was built and launched by a consortium of more than 100 scientists and engineers from 11 countries.

“The planet WASP-189b was detected in 2018; because of its unusual orbit close to its central star, we studied it with CHEOPS very early on,” says Szilárd Csizmadia from the German Aerospace Centre.

“The precise measurements made with CHEOPS have now revealed its extraordinary characteristics: it is an ultra-hot planet, almost 1.6 times the diameter of Jupiter, and its orbit around its star is strangely inclined.”

WASP-189b is only 7.5 million kilometres from its star (Earth is 150 kilometres from the Sun) and an orbit takes just 2.7 days to complete. Its star is larger and over 2000 degrees hotter than the Sun, and therefore appears to glow blue.

“Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” says Lendl. “WASP-189b is also the brightest hot Jupiter that we can observe as it passes in front of or behind its star, making the whole system really intriguing.”

The star itself is not a perfect sphere; it rotates so fast that it deforms. Its equatorial radius is thus greater than its polar radius, causing it to be cooler at the equator and hotter at the poles, and the poles to appear brighter.

Also intriguing, the researchers say, is that WASP-189b’s orbit is not in the equatorial plane of the star, as would be expected if the star and planet developed from a common disc of gas and dust that passes on its rotational direction to its planets, as is the case in the Solar System. The orbit of WASP-189b, however, passes over the poles of its star.

Planetary objects like WASP-189b are exotic, Lendl says, because they have a permanent day side, which is always exposed to the light of the star, and, accordingly, a permanent night side.

This means that its climate is completely different from that of Jupiter and Saturn, the gas giants in our Solar System.