A Distant Blue Star Hosts One of the Most Extreme Exoplanets Known to Science

The newly deployed CHEOPS space telescope has completed its first observations of an exoplanet, uncovering some fascinating new details about an ultra-hot Jupiter known as WASP-189b.

Artistic impression of exoplanet WASP-189b orbiting its host star, which glows in blue.

© Image: ESA
Artistic impression of exoplanet WASP-189b orbiting its host star, which glows in blue.

Hot Jupiters are Jupiter-like exoplanets located in close proximity to their host stars, hence their name. Ultra-hot Jupiters are basically the same thing, but, as you’ve probably guessed, they’re even hotter. Back in 2018, astronomers using the ground-based WASP-South telescope in South Africa detected an ultra-hot Jupiter dubbed WASP-189b, unlike anything seen before.


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Two years later, using the brand-spanking-new Characterising Exoplanet Satellite (CHEOPS) space telescope, astronomers have gazed upon this celestial wonder with new eyes, refining what we know of this unusual exoplanet, while at the same time affirming the tremendous potential of this European space telescope, which only began making scientific observations this past April.

Indeed, CHEOPS, when compared to ground-based telescopes, “is simply much more precise,” Monika Lendl, an astronomer at the University of Geneva and the lead author of the new study, explained in an email. “Because CHEOPS observes from space, it doesn’t need to look through the Earth’s atmosphere, and so the light doesn’t get disturbed by air turbulence.”

CHEOPS, a collaboration between the European Space Agency and the Swiss Space Office, is solely designed to detect and observe exoplanets, which it does by spotting dips in a star’s brightness—a potential sign of an exoplanet passing in front (i.e. the transit method of detection). CHEOPS will also study previously detected exoplanets, as is the case here with WASP-189b.

“Cheops has a unique ‘follow-up’ role to play in studying such exoplanets,” Kate Isaak, a CHEOPS project scientist at ESA and a co-author of the new study, said in a press release. “It will search for transits of planets that have been discovered from the ground, and, where possible, will more precisely measure the sizes of planets already known to transit their host stars.”

A new paper describing the space telescope’s first formal investigation has been published to Astronomy & Astrophysics.

WASP-189b is located 322 light-years away in the Libra constellation of the southern hemisphere. This ultra-hot Jupiter is in a tight orbit around A-type star HD 133112, which glows in blue. The exoplanet, at just 4.7 million miles (7.5 million km) from its host star, requires just 2.7 days to make a complete orbit. Given this cozy arrangement—at about 5% the distance of Earth to the Sun—WASP-189b is very hot, with the new CHEOPS data refining previous estimates.

diagram, schematic

© Image: ESA

The temperature of WASP-189b is actually tough to figure out, because this gas giant is quite bright, causing conflicting data between itself and its host star. To get around this, Lendl and her colleagues waited for occultations, in which planets pass behind their host stars from our perspective (kind of like the opposite of the transit method). This allowed the scientists to properly discern the exoplanet’s brightness, which in turn allowed them to take its temperature.

“WASP-189b is one of the hottest gas giants known to exist,” said Lendl. “With CHEOPS, we have been able to determine the brightness of the planetary dayside and find that the light emitted by it corresponds to a planet with a temperature of about 3,200 degrees Celsius [5,800 degrees Fahrenheit].”

That’s intense; our Sun is just 2,000 degrees C (3,630 degrees F) hotter than this scorching exoplanet. And in fact, WASP-189b is actually hotter than some red dwarf stars, which cook along at temperatures well below 3,000 degrees C (5,430 degrees F). The chances of any life existing on this planet are basically nil, as even iron converts to gas at these extremes.

Few planets are known to be this hot. WASP-189b is also the brightest hot Jupiter known to scientists.

The researchers refined the exoplanet’s mass, finding that it’s almost exactly two times heavier than Jupiter. They also updated WASP-189b’s diameter, finding that it’s 1.6 Jupiter’s width, or 139,000 miles (224,000 km), which is slightly bigger than previous calculations.

The scientists also noticed that the star, HD 133112, isn’t perfectly round—it’s actually kind of squished, bulging at the equator, where it’s noticeably cooler compared to the polar regions. The star’s rapid spinning and resulting centrifugal tidal forces are contributing to its odd shape, note the authors in the study.

Interestingly, WASP-189b is in an inclined orbit, which means it’s out of line with the star’s equatorial plane. Actually, it’s really misaligned, zooming above the star’s polar regions. This is an important observation, as it means the exoplanet likely formed farther out and then slowly migrated inward over time. This trek towards the host star happened either due to the gravitational influence of other planets in the same system or the influence of another star, the researchers speculate.

With its observations of WASP-189b complete, CHEOPS will now turn its attention to hundreds of other known exoplanets and their host stars, which it will do to further constrain their mass, size, and orbits. As this inaugural investigation makes clear, we can expect much more from this exciting new space telescope.

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