The newly discovered sweltering planet has the shortest orbit of any known gas giant.

The hunt for planets beyond our solar system, meanwhile, has led to the discovery of more than 4,000 distant worlds orbiting stars thousands of light-years away. These exoplanets form a diverse melange; from rocky super-Earths, to miniature Neptunes and colossal gas giants. Among the most exotic are astronomers so-called ‘hot Jupiters’. And now researchers in a new study discovered a very strange specimen.

More about hot Jupiters
Hot Jupiters are massive spheres of gas about the size of planet Jupiter in our own solar system. But — unlike Jupiter’s plodding 12-year orbit around our sun — hot Jupiters complete one orbit around their star in less than 10 days. To date, scientists have discovered about 400 hot Jupiters. How exactly these curious gas giants form, however, remains one of the greatest unsolved mysteries in planetary science.

With the help of planet hunter Tess, researchers tracked down the hot Jupiter. The planet is located about 855 light-years away from Earth, located in the southern part of the constellation Hercules. Then by analyzing different optical and infrared wavelengths, the team determined that the planet — which has been named TOI-2109b — is about five times as massive as Jupiter and about 35 percent larger.

Orbit

The most remarkable thing, however, is its particularly tight orbit. The researchers found that TOI-2109b is only 2.4 million kilometers away from its parent star. By comparison, Mercury’s distance from the sun is about 58 million kilometers. This means that TOI-2109b orbits its star extremely short, even having the shortest orbit of all known gas giants. A year on TOI-2109 only lasts 16 hours! In other words, on this planet it is New Year’s Day every 4 o’clock.

The starlight of TOI-2109 measured by TESS shows periodic dips in brightness. This reveals that a planet passes in front of the star every 16 hours. Image: MIT

Due to its short orbital period, temperatures on the day side of TOI-2109b rise bizarrely high. Just as we always see the same side of the moon, a hot Jupiter always has the same side facing its star. This means that it is always day on one side of the planet and permanently night on the other side. Now, researchers have found that the mercury on the dayside of TOI-2109 can rise as high as a scorching 3,226 degrees Celsius. That’s really blazing hot, hotter than a small star. With these temperatures, TOI-2109b is the second hottest planet discovered to date.

Orbital Decay

Judging by its curious features, astronomers suspect TOI-2109b is in ‘orbital decay’. This means that it is slowly but surely spiraling towards its parent star, similar to how bathwater flows down the drain. The researchers think that the extremely short orbit of the hot Jupiter will cause the planet to move faster and faster towards its star; faster than any other known hot Jupiter. And that’s very interesting. The discovery of TOI-2109b offers researchers a unique opportunity to study how planets behave when they are attracted and swallowed by their parent star. “If we’re lucky, we could see the planet getting closer to its star in a year or two,” said researcher Ian Wong. “Unfortunately, in our lifetime, we will not see the planet swallowed by its star. But give it another 10 million years, and this planet is probably gone.”

Extreme subclass

The researchers hope to observe TOI-2109b in the near future with more powerful instruments, including the Hubble Space Telescope and the soon-to-be-launched James Webb Telescope. The team can’t wait. “Ultra-hot Jupiters like TOI-2109b are the most extreme subclass of exoplanets,” Wong explains. “We’re actually just beginning to understand some of the unique physical and chemical processes that take place in their atmospheres — processes unknown in our own solar system.”

Furthermore, future observations of TOI-2109b could provide clues as to how such staggering systems might arise in the first place. “Since the dawn of exoplanetary science, hot Jupiters have been seen as the odd one out,” said researcher Avi Shporer. “How does a planet as massive and large as Jupiter enter an orbit that lasts only a few days? We don’t know anything like this in our own solar system. And so we see the discovery of TOI-2109b as an opportunity to study these extraordinary worlds and explain their existence.”