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Astronomers have discovered an exoplanet with a very long orbit that experiences extreme temperature changes that may be transitioning into a different type of world.
The exoplanet, named TIC 241249530 b, orbits a star about 1,100 light-years away from Earth. The star is part of a binary star system, meaning the planet orbits the primary star, which in turn orbits the secondary star.
An interaction between two stars with misaligned orbits may have put the planet on the path to becoming a “hot Jupiter,” researchers reported in a study published in the journal Nature on Wednesday. Nature.
Astronomers have identified more than 5,600 exoplanets, of which between 300 and 500 are “hot Jupiters” — giant gas bodies like Jupiter that orbit their host stars and are scorching hot from their heat.
It takes Jupiter 4,000 Earth days to complete one revolution around the Sun, but Hot Jupiter completes an orbit every few days.
Scientists think that these massive planets start out orbiting their stars from afar, but move closer over time. But it’s long been a question of how the giant planets end up orbiting so close to their stars, much closer than Mercury does.
Observations of TIC 241249530 b, first imaged in January 2020 by NASA’s TESS satellite, are providing rare and revelatory insights into what may be a planet on its way to becoming a hot Jupiter.
“For more than 20 years, astronomers have been searching for exoplanets that could be precursors of hot Jupiters or intermediate products in the migration process, so we were extremely surprised and excited by its discovery,” Arvind Gupta, a postdoctoral researcher at NOIRLab and lead author of the study, who discovered the planet as a doctoral student at Penn State, said in a statement.
On January 12, 2020, the Transiting Exoplanet Survey Satellite collected data that suggests something is passing in front of its host star, TIC 241249530. TESS monitors the brightness of nearby stars, looking for drops in their light that could indicate the presence of an exoplanet.
Gupta and his colleagues sifted through the data and determined that a Jupiter-sized planet was passing in front of the star, then used instruments on the WIYN 3.5-meter telescope at Kitt Peak National Observatory in Arizona to measure the star’s radial velocity, or how much the star wobbles back and forth as the planet’s gravity tugs on it.
Radial velocity data also confirmed the existence of the same planet, enabling researchers to determine that it has a mass roughly five times that of Jupiter and has what astronomers call a highly eccentric orbit.
Astronomers use “eccentricity” to describe the shape of a planet’s orbit on a scale of 0 to 1, with 0 corresponding to a perfectly circular orbit. In our solar system, Earth’s eccentricity is 0.02, while Pluto’s highly elliptical orbit around the Sun is 0.25.
The newly discovered exoplanet has an eccentricity of 0.94, making it more elliptical than any exoplanet astronomers have found to date, according to the researchers. The strange planet takes about six months to orbit its star, getting very close before darting out and then returning to a tight, elliptical orbit shaped like a cucumber.
“We captured this giant planet making a sharp hairpin turn on its way close to its star,” Subrath Mahadevan, the Vern M. Willaman Professor of Astronomy at Penn State and co-author of the study, said in a statement. “Such highly eccentric transiting planets are extremely rare, so finding one of the most eccentric is truly remarkable.”
The planet is just 3 million miles from its star, more than 10 times closer than Mercury gets to the Sun (for comparison, Mercury is an average of 36 million miles (58 million kilometers) from the Sun). National Aeronautics and Space Administration (NASA).
This extreme orbit causes “large temperature fluctuations” throughout the planet’s year, said Jason Wright, a professor of astronomy and astrophysics at Penn State and co-author of the study.
“For a few days as the moon passes close to the star, cloud-top temperatures become hot enough to melt titanium,” Wright said in an email. “For most of its orbit, the moon is farther away, and at its farthest point, daytime cloud-top temperatures are similar to a warm summer’s day on Earth.”
The team also discovered that the planet orbits backwards, or moves in the opposite direction, compared to the star’s rotation — a rare phenomenon not seen in most exoplanets, and not even in our own solar system.
All of the oddities observed about TIC 241249530 b are helping astronomers understand how the planet formed.
“While we can’t hit the rewind button and watch the planetary migration process in real time, this exoplanet serves as a sort of snapshot of the migration process,” Gupta said. “Planets like this are hard to find, and we hope they will help elucidate the story of hot Jupiter formation.”
To understand why the planet ended up on such an unusual orbit and how it might evolve over time, the team ran simulations, which included modeling the gravitational interactions between TIC 241249530 b and its primary and secondary stars.
The team estimated that the planet probably formed far from its star and began in a wide, circular orbit similar to Jupiter’s, but the star’s orbit was misaligned with that of the second star, which exerted gravitational forces on the planet, causing it to expand, the researchers said.
“After many orbits, the gravitational influence of the outer star changes TIC 241249530 b’s orbit, causing it to become increasingly elongated,” Wright said.
As the star loses energy each time the planet passes through its orbit, astronomers estimate that in a few hundred million years, the orbit will shrink and stabilize, lasting just a few days instead of the current 167 days.
That would make the planet a true hot Jupiter, said Sarah Milholland, an assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Sciences and a co-author of the study.
“This is a pretty extreme process in that the changes in the planet’s orbit are quite large,” Milholland said. “These are massive orbital changes that take place over billions of years, and the planet is just going along with it.”
The only other early hot Jupiter known before TIC 241249530 b was HD 80606bIt was discovered in 2001. HD 80606 b was thought to be the most eccentric planet until its recent discovery.
HD 80606 b has an eccentricity of 0.93, a short orbital period of 111 days, and orbits in the same direction as its star, but otherwise the two planets are effectively twins, Wright said. Finding two planets at such a short stage in the planet’s orbital evolution is “like accidentally coming across a butterfly just as its chrysalis opens,” Wright said.
The researchers said the discovery of a second hot Jupiter progenitor could help astronomers confirm the idea that high-mass gas giants transform into hot Jupiters as they transition from highly eccentric to more circular orbits.
The team plans to observe TIC 241249530 b with the James Webb Space Telescope to characterize the dynamics of its atmosphere and observe how it responds to rapid heating, while continuing the search for planets like these that transform into hot Jupiters.
“This system highlights how amazingly diverse exoplanets are,” Milholland said. “They’re mysterious, other-worldly worlds, and their orbits tell us how they got there and where they’re going — and in this case, that journey isn’t quite over yet.”
