A New Hot Jupiter

Photo of TOI-2109b courtesy of NASA

Discovering Exoplanet TOI-2109b

Looking out my west-facing bedroom window on a clear, dark night, I see hundreds of stars, each just a point of light. Many of those stars may be like our sun with a system of orbiting planets. We’ve long suspected that other suns have other planets. Back in 1979, Gordon Walker began searching for planets at the University of British Columbia. In 2019, Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland won the Nobel prize for their 1995 discovery of the gas giant Dimidium 50 light years distant. 

Our planets, Jupiter and Saturn, are also gas giants. Hydrogen and helium are the major components of these large planets. Gas giants don’t have rocky land masses or liquid water oceans, so there is no “surface” for a spacecraft to land on. Many gas giants, like our Jupiter, have a thick mantle of gas swirling above a denser core. However, other gas giants in other systems differ greatly from the two in our solar system.

New Hot Jupiter

In November 2021, NASA announced the discovery of a new gas giant that orbits a star, 860 light years away from our solar system. This planet, TOI-2109b, is about one-third larger than Jupiter. However, it has five times the mass, making it much denser. This newly discovered exoplanet is hot, fast, and doomed.

Our Jupiter is an inhospitably chilly place. Orbiting at 484 million miles, Jupiter gets very little radiant energy from our Sun. Its temperature is -234 degrees F (or -145 C), just a few degrees above the point where hydrogen becomes a liquid, according to NASA. Mercury, the planet closest to our Sun at 36 million miles, engulfed in radiant energy, reaches 800 F. In contrast, the sunny side of TOI-2109b tops out at over 6000 degrees F since it orbits less than 2 million miles from its star.

Like our Moon, TOI-2109b rotates on its axis only once every orbit. Since TOI-2109b always shows the same side to the earth, its sunny side always faces its star. This is also part of the reason this “sunny” side is so hot.

“The temperatures on this planet even exceed those of some stars,” says Ian Wong, an astronomer at NASA Goddard Space Flight Center in Maryland. Astronomers call planets like TOI-2109b “Hot Jupiters.” “If you look at the whole planetary demographics that we’ve discovered in the past decade,” continues Wong, “we know that these Hot Jupiters are pretty rare.” The few discovered fascinate astronomers. [Discover Magazine]

This new Hot Jupiter also has a very short year, since it zips around its star, completing one round trip in about 16 earth hours. In comparison, our Jupiter takes 12 earth-years to circle our Sun and Mercury, the closest planet, takes 88 earth day. 

Both our Jupiter and TOI-2109b, however, spin speedily on their axes. A day on Jupiter, one rotation on its axis, is only 10 earth hours long, while a day on TOI-2109b is 16 earth hours long. Some astronomers think massive gas giants like our Jupiter and TOI-2109b are whirling at the ragged edge of their break-up velocity–any faster and they might tear apart.

Future observations of TOI-2109b may also reveal clues about the origins of such dizzying planetary systems. “From the beginning of exoplanetary science, we have seen Hot Jupiters as oddballs,” Avi Shporer of MIT says. “How does a planet as massive and large as Jupiter reach an orbit that is only a few days long? 


TOI-2109b is also a doomed planet. Its star is slowly dragging it inward to a fiery end. A massive planet, like TOI-2109b, orbiting very close to its star, exerts a large gravitational pull on the star, just as the star pulls on the planet. Star and planet drag each other from a typical spherical shape to more of a distorted oval shape. This tug-of-war will end with TOI-2109b plummeting into its star. “We won’t see this happen in our lifetime,” according to Wong at NASA, but maybe in 10 million years this star will consume its close companion, this Hot Jupiter.

Big Planets, Big Questions

These Hot Jupiters raise questions as large as these planets themselves. Do gas giants form far from their stars and then draw closer? Or did they form that close? If gas giants can migrate, what might that mean for our solar system? What happens when a star consumes a planet? For scientists to answer questions like these, they must observe many more of these hot gas giants as different stages in their formation. 

“We have nothing like this in our solar system, and we see this as an opportunity to study them and help explain their existence,” Shporer says.

The study of various types of exoplanets will help us learn how the universe formed, along with its history and its destiny. “Ultrahot Jupiters such as TOI-2109b constitute the most extreme subclass of exoplanet,” Wong says. “We have only just started to understand some of the unique physical and chemical processes that occur in their atmospheres — processes that have no analogs in our own solar system.”

So far, we have discovered over 4000 exoplanets and astronomers look for several clues as they gaze at the cosmos. For example, the mutual gravitational bond between the massive TOI-2109 and its star causes the star to “wobble” when viewed from a distance. They also look for the slight dimming of the star when an exoplanet passes between the star and the earth, much like a partial eclipse. 

The hunt for exoplanets isn’t just for professional astronomers. Citizen astronomers have spotted exoplanets with their home equipment and student astronomers have also made discoveries. For example, in 2021, high school senior Jasmine Wright and junior Kartik Pingle discovered not one, but four new exoplanets while participating in a research program through the Harvard & Smithsonian Center for Astrophysics. If that sounds exciting, you too could explore new worlds, feet planted on the earth, but with eyes turned to the stars.

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  • Discover Magazine
  • MIT News Office
  • NASA
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