The search for planets outside our solar system has revealed more than 4,000 distant worlds, orbiting stars thousands of light years from Earth. These extrasolar planets are a veritable zoo, from super-rocky Earths and miniature Neptunes to giant gas giants.
Among the most puzzling planets discovered so far is “hot Jupiter” – huge balls of gas the size of our own Jovian planet but orbiting their stars in less than 10 days, unlike Earth. Slow Jupiter, 12-year orbit. Scientists have discovered about 400 hot Jupiters so far. But exactly how these weight whirlwinds arose remains one of the greatest unsolved mysteries in planetary science.
Today, astronomers have discovered one of the hottest extremes of Jupiter – a gas giant with a mass about five times the mass of Jupiter that orbits its star in just 16 hours. The planet’s orbit is the shortest of any gas giant known to date.
Due to its extremely narrow orbit and proximity to its star, the planet’s diurnal side is estimated to be about 3,500 K, or roughly 6,000 degrees Fahrenheit, or roughly the heat of a small star. This makes the planet, designated TOI-2109b, the second hottest planet discovered to date.
Based on its properties, astronomers believe TOI-2109b “disintegrates into orbit,” or billows toward its star, like the shower waters surrounding the sink. Its extremely short orbit should spin the planet toward its star faster than other hot Jupiters.
The discovery, initially made by NASA’s Transiting Exoplanet Survey Satellite (TESS), a mission led by the Massachusetts Institute of Technology, presents a unique opportunity for astronomers to study how planets behave as they are pulled and swallowed by their star.
“Within a year or two, if we’re lucky, we might be able to figure out how the planet approaches its star,” says lead author Ian Wong, who was a postdoctoral researcher at MIT during the study. Goddard Space Flight Center for NASA. “In our lifetime, we will never see the planet fall into its star. But give it another 10 million years, and this planet may be gone.”
The discovery was reported today in astronomical diaries It is the result of extensive collaborative work involving members of the TESS science team at MIT and researchers from around the world.
On May 13, 2020, NASA’s TESS satellite began observing TOI-2109, a star located in the southern part of the constellation Hercules, about 855 light-years from Earth. The mission identified the star as “TESS Object of Interest” number 2,109, for the possibility that it could host a planet orbiting it.
For about a month, the spacecraft collected starlight measurements, which the TESS science team then analyzed for transits – periodic dips in starlight that could indicate a passing planet. In front of her, he briefly blocked out a small portion of the starlight. TESS data confirmed that the star actually hosts something that passes about every 16 hours.
The team informed the broader astronomical community, and soon several ground-based telescopes followed over the next year to closely observe the star across a range of frequency bands. These observations, along with the initial discovery by TESS, confirmed that the transiting object was a planet orbiting the planet, named TOI-2109b.
” Everything was consistent with the fact that it’s a planet, and we realized we had something very interesting and relatively rare,” says study co-author Avi Spurer, a researcher at the Kavli Institute for Astrophysics and Space Research at MIT.
day and night
By analyzing measurements at various optical and infrared wavelengths, the team determined that TOI-2109b is five times larger than Jupiter, about 35% larger, and very close to its star, at a distance of about 2.5 million km. By comparison, Mercury is about 36 million km from the Sun.
The planet’s star is about 50% larger in size and mass than our sun. Based on the observed characteristics of the system, the researchers estimated that TOI-2109b was moving toward its star at a rate of 10 to 750 milliseconds per year, faster than any hot Jupiter observed so far.
Given the planet’s dimensions and proximity to its star, the researchers determined that TOI-2109b was an extremely hot Jupiter, with the shortest orbit of any known gas giant. Like most hot Jupiters, the planet appears to be progressively locked, and has a perpetual day and night aspect, similar to the Moon relative to the Earth. From TESS’s month-long observations, the team was able to observe the planet’s changing brightness as it rotated on its axis. By observing the planet passing behind its star (called a secondary eclipse) at optical and infrared wavelengths, the researchers estimated that the day side reaches temperatures of more than 3,500 K.
“In the meantime, the luminosity of the night side of the planet is less than sensitive to the TESS data, which raises questions about what’s really going on there,” said Sporer. “Is the temperature there too cold, or does the planet somehow take the heat from the day and transfer it to the night side? We’re starting to try to answer that question for these super-hot Jupiters.”
Researchers hope to monitor TOI-2109b with more powerful instruments in the near future, including the Hubble Space Telescope and the soon-to-be-launched James Webb Space Telescope. More detailed observations could shed light on the conditions that hot Jupiters go through when they fall into their star.
“Ultra-hot Jupiters like TOI-2109b are the most extreme subclass of exoplanets,” Wong says. “We are just beginning to understand some of the unique physical and chemical processes that occur in the atmosphere — processes that have no analogues in our solar system.”
Future observations of TOI-2109b could also reveal clues about how to create such amazing systems. “Since the beginning of exoplanet science, hot Jupiters have been considered exotic spheres,” says Sporer. “How does a massive and massive planet like Jupiter reach an orbit that only lasts a few days? We don’t have anything like that in our solar system, and we see this as an opportunity to study it and help explain its existence.”
This research was supported, in part, by NASA.
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