Scientists take the temperature of an extreme alien world

An artist's impression of WASP-189b orbiting its host star - (c) ESA

A European Space Agency (ESA) mission involving a Keele astrophysicist, which has been searching for exoplanets, has observed one of the hottest and most extreme extra-solar planets discovered to date - its first findings since launching.

The planet, known as WASP-189b, is a so-called ultra-hot Jupiter – a gas planet similar to Jupiter in our own solar system, but which orbits much closer to its host star meaning it is heated to extreme temperatures.

Keele’s Dr Pierre Maxted is a co-author on both the current CHEOPS study, and also co-authored the publication of the discovery of WASP-189b.

WASP-189b sits around 20 times closer to its star than Earth does to the Sun and completes a full orbit in just 2.7 days. Its host star is larger and more than 2000 degrees hotter than the Sun, and so appears to glow blue. The planet also has an inclined orbit, meaning it travels close to the star’s poles rather than around its equator.

This is the first exoplanet observation made as part of the ESA’s ongoing Characterising Exoplanet Satellite (CHEOPS) mission, which is made up of an international team of academics from universities including Keele, searching nearby stars known to host planets. These findings have been published in the journal Astronomy and Astrophysics.

Dr Maxted from Keele’s School of Chemical and Physical Sciences, who is part of the mission’s research team said: “WASP-189b is a system at the limit of what we can discover using the ground-based WASP instruments. With CHEOPS we can measure the properties of this systems to amazing precision. This first science result from CHEOPS demonstrates how well this spacecraft is working so I’m excited to see what we will discover using this unique instrument.”

Lead author Dr Monika Lendl of the University of Geneva in Switzerland said: “Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest. 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 researchers studied the planet by observing it passing behind its host star – an event know as an occultation – as well as studying it passing in front of the host star – known as a transit.

Using the occultation, the researchers were able to measure the planet’s brightness and temperature which they recorded at a scorching 3200 degrees C. This makes WASP-189b one of the hottest and most extreme planets discovered, and entirely unlike any of the planets of our Solar System. At such temperatures, even metals such as iron melt and turn to gas, making the planet a clearly uninhabitable one.

Studying a planetary transit can also reveal much about the size, shape, and orbital characteristics of a planet. This was true for WASP-189b, which was found to be larger than thought at almost 1.6 times the radius of Jupiter.

This was the first exoplanet studied using the CHEOPS satellite, which launched in December 2019. The probe has been working to expand our understanding of exoplanets and the nearby cosmos in the months since.

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