Astronomers Find a Planet That Orbits its Star in Just 16 HOURS!
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Astronomers Find a Planet That Orbits its Star in Just 16 HOURS!

Mercury is the speed champion in our Solar System. It orbits the Sun every 88 days at a speed of 47 km/s. Its average distance from the Sun is 58 million kilometers (36 million miles), and it travels so quickly that it was named after Mercury, the wing-footed God.

But what if Jupiter, instead of Mercury, was closest to the Sun? What if Jupiter was even closer to the Sun and much hotter than Mercury?

There’s a planet in a remote solar system some 855 light-years away that makes Mercury appear like a slow, chilled, distant neighbor of the Sun. This planet takes only 16 hours to orbit its star, giving it one of the shortest orbits ever measured. It’s one of the most exotic planets ever discovered, given its distance and speed, as well as its exceptionally high surface temperatures.

TOI-2109b is the name of the planet, and it is classified as a “Ultrahot Jupiter” by astronomers. Hot Jupiters are gas giants that orbit extraordinarily close to their stars and have extremely high surface temperatures. Ultrahot Jupiters are significantly hotter. Their surface temperatures exceed 2200 Kelvin (1900 C, 3500 F). TOI-2109 b’s dayside temperature is estimated to be greater than 3500 K (3225 C, 5840 F), making it as hot as certain tiny stars.

A paper published in The Astronomical Journal presents the discovery. The paper’s title is “TOI-2109: An Ultrahot Gas Giant on a 16 hr Orbit.” Ian Wong, who is currently at NASA’s Goddard Space Flight Center but was a postdoc at MIT during this research, is the lead author.

NASA’s TESS (Transiting Exoplanet Survey Satellite) of NASA discovered the planet in May 2020. TESS began observing it on May 13th and continued to do so for nearly a month. Several ground-based observatories conducted follow-up observations at various wavelengths over the next year. TOI-2109b is a rare and unusual planet, according to all of the observations.

“Everything was consistent with it being a planet, and we realized we had something very interesting and relatively rare,” said study co-author Avi Shporer from MIT’s Kavli Institute for Astrophysics and Space Research.

TOI-2109 b’s orbital period of 16 hours is the shortest yet measured for a gas giant. (The previous record-holder had an orbit that lasted 18 hours.) The planet is nearly five times the mass of Jupiter and orbits an F-type star about 1.5 times the mass of our Sun. It’s difficult to imagine how this arrangement would appear to some other observer in the same system.

Artist’s impression of a transiting Jupiter-mass exoplanet around a star that’s slightly more massive than the Sun. Image Credit: NASA

The planet is so hot because it is only 2.4 million km (1.5 million miles) from its star. Like other Hot Jupiters and Ultrahot Jupiters, it is most likely tidally locked to its star. The unusually high daytime temperature can break molecules into atoms. Theoretical modelling shows that this can happen to molecular hydrogen. The hydrogen can assemble into molecules again if the night side is much cooler.

After a month of TESS observations, the team was able to view the planet as it orbited its star. They observed the secondary eclipse, which occurs when a planet passes behind its star, in several wavelengths. This helped them determine that the daytime temperature was most likely higher that 3500 K. But because TESS isn’t sensitive enough, the researchers don’t know what happens at night. If it’s true that molecular hydrogen is torn apart on the dayside and recombines on the nightside, then that could contribute to more efficient temperature mixing in the atmosphere and could mean the temperature isn’t as extreme.

“Meanwhile, the planet’s night side brightness is below the sensitivity of the TESS data, which raises questions about what is really happening there,” said Shporer. “Is the temperature there very cold, or does the planet somehow take heat on the day side and transfer it to the night side? We’re at the beginning of trying to answer this question for these ultrahot Jupiters.”

TOI-2109b is slowly spiralling into the star at a rate of 10 to 750 milliseconds each year, according to the researchers. Other Hot Jupiters have been found whose orbital decay pushes them into their stars, but none as quickly as this.

Artist’s concept of Jupiter-sized exoplanet that orbits relatively close to its star (aka. a “hot Jupiter”). Credit: NASA/JPL-Caltech)

The extreme nature of TOI-210 b supports the classification of Ultrahot and Hot Jupiters as one of the most extreme types of exoplanets. More powerful telescopes will reveal more about the planet’s nature, and the team believes that the Hubble Space Telescope, as well as the soon-to-be-launched James Webb Space Telescope, will be able to investigate it. Astronomers are especially interested in what happens as the planet gets closer and closer to the star.

“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.”

Future observations of TOI-2109b may also reveal clues to how such dizzying systems came to be in the first place. “From the beginning of exoplanetary science, hot Jupiters have been seen as oddball,” Shporer says. “How does a planet as massive and large as Jupiter reach an orbit that is only a few days long? We don’t have anything like this in our Solar System, and we see this as an opportunity to study them and help explain their existence.”

Jupiter may have moved to near 1.5 AU of the Sun in the distant past before reversing course to the orbital path it now follows. This is known as the Grand Tack Hypothesis. That would have been something to see with human eyes.

Jupiter migrated toward the Sun shortly after its formation. Saturn did as too, and their fates eventually became linked. The couple turned and travelled away from the Sun when Jupiter was roughly where Mars is now. Scientists have called this the “Grand Tack,” after the sailing maneuver. Credit: NASA/GSFC

Finding extreme and unusual exoplanets teaches us a lot about the various types of planets that exist. Hot Jupiters and Ultrahot Jupiters are common in exoplanet surveys because they are massive and close to their sun. However, they are actually scarce.

The scientists point out that these extreme planets are found in just approximately 0.5 percent of Sun-like stars. Despite their small number, they offer a significant contribution to our total understanding of exoplanets. “Their large size in relation to their host stars and high temperatures enable a broad range of intensive studies that extend far beyond the rudimentary measurements of planet mass and radius,” the authors explain.

“Over the past two decades, a wide arsenal of observational techniques has been leveraged to probe the atmospheric properties of hot Jupiters in ever-increasing detail,” they write in their paper. Things like temperature distribution, chemical composition, condensate clouds, photochemical hazes, and heat transport mechanisms are becoming easier to study.

Astronomers are learning that Ultrahot Jupiters are “… characterized by a number of distinct physical and dynamical properties that set them apart from the rest of the hot gas-giant population.”

No exoplanet article would be complete without mentioning the James Webb Space Telescope. The JWST will be able to probe exoplanet atmospheres more thoroughly than any other tool currently available to astronomers.

Finding Earth-like planets in habitable zones is an essential part of the search for and study of exoplanets. However, ultrahot Jupiters such as TOI-2109b can teach us a lot about planets at their most extreme, as well as planet-star interactions that we cannot study in our Solar System. And the JWST will make a significant contribution to our knowledge.

“While future advances in telescope capabilities will enable comparably in-depth explorations of smaller and cooler exoplanets,” the authors write, “ultrahot Jupiters will continue to be among the most fruitful candidates for impactful characterization efforts, providing crucial insights into the nature of planets at their most extreme.”

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