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Ultra-hot Jupiters are an odd, hellish class of

exoplanets that astronomers are increasingly

finding scattered throughout the cosmos. These

gas giants sit much closer to their host stars than

Mercury does to the Sun, often leading to tidal

locking — the same side of the planet always

faces the star. Their dayside temperatures can

exceed 3,500 degrees Fahrenheit (1,900 degrees

Celsius), while nightside temperatures hover

around 1,800 F (1,000 C).

In a study accepted for publication in

Astronomy and Astrophysics, researchers mod-

eled the atmospheres of four known ultra-hot

Jupiters that were previously investigated using

the Hubble and Spitzer space telescopes. The

team concluded that ultra-hot Jupiters are even

more two-faced than astronomers initially

thought. Specifically, the team found that the

dayside temperature of these exoplanets is so

intense that the heat can split most types of

molecules into their basic building blocks. Since

these molecules are broken apart, they remain

hidden from the gaze of even our most

advanced observatories. This led the research-

ers to a surprising conclusion: The dayside

atmosphere of an ultra-hot Jupiter more closely

resembles a star than a planet.

Although this result is interesting, it also

helps explain why astronomers only detect

water molecules at the day-night border of

ultra-hot Jupiters. The team found that as

hydrogen and oxygen atoms make their way to

the planet’s cooler nightside, they recombine

to form water. But since the nightside is too

dark to observe, astronomers can only detect

these water molecules right at the border

between day and night.

This new research provides a valuable

framework that will help astronomers better

understand the physical processes that govern

these mold-breaking worlds. And with NASA’s

planet-hunting TESS telescope already collect-

ing data, the more we know, the better. — J.P.

Ultra-hot Jupiters have

daysides like stars

MELTING POT. This illustration depicts an ultra-hot

Jupiter that sits so close to its host star, its dayside

atmosphere is hot enough to boil iron.

ENGINE HOUSE VFX, ATBRISTOL SCIENCE CENTRE, UNIVERSITY OF EXETER

P

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