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Astronomers have detected an exoplanet with a very elongated orbit that experiences large temperature variations – and it could be transitioning to another type of world.
The exoplanet, named TIC 241249530 b, orbits a star about 1,100 light-years from Earth. The star is part of a binary pair, meaning the planet orbits the primary star, while the primary star orbits a secondary star.
Interactions between the two stars, which have misaligned orbits, could be responsible for putting the planet on track to become a “hot Jupiter,” researchers reported in a study published Wednesday in the journal Nature.
Astronomers have discovered more than 5,600 confirmed exoplanets, 300 to 500 of which are “hot Jupiters.” These planets are massive, gaseous bodies similar to Jupiter that orbit their host star, heating them to scorching temperatures.
While Jupiter takes 4,000 Earth days to complete one orbit around the Sun, hot Jupiters complete one orbit every few days.
Scientists believe that giant planets first orbit their stars at a certain distance and then gradually move closer together. But they have long wondered how these massive planets end up in such tight orbits, which are much closer to their stars than Mercury is to our Sun.
Observations of TIC 241249530 b, first captured by NASA’s TESS planet-finding satellite in January 2020, offer rare and revealing insights into what may be a planet on its way to becoming a hot Jupiter.
“Astronomers have been searching for more than two decades for exoplanets that are likely precursors to hot Jupiters, or are intermediate products of the migration process, so I was very surprised – and excited – to find one,” study lead author Arvind Gupta, a NOIRLab postdoctoral researcher who discovered the planet while a doctoral student at Penn State, said in a statement.
On January 12, 2020, the Transiting Exoplanet Survey Satellite collected data suggesting that something was passing in front of the host star TIC 241249530. TESS monitors the brightness of nearby stars to look for dips in brightness that could indicate the presence of exoplanets.
Gupta and his colleagues analyzed the data and determined that a Jupiter-sized planet was passing in front of the star. They then made measurements using instruments on the 3.5-meter WIYN telescope at Kitt Peak National Observatory in Arizona to determine the star’s radial velocity, or how much the star wobbles back and forth as the planet’s gravity pulls on the star.
Radial velocity data also confirmed the presence of the same planet and helped researchers clarify that it was about five times more massive than Jupiter and had what astronomers call a highly eccentric orbit.
Astronomers use the term “eccentric” to refer to the shape of a planet’s orbit on a scale from zero to 1. Zero is a perfectly circular orbit. In our solar system, Earth has an eccentricity of 0.02, while Pluto’s very oval orbit around the Sun is considered to be 0.25.
The newly discovered exoplanet has an eccentricity of 0.94, which is more oblong than any other transiting exoplanet astronomers have ever discovered, the researchers said. The strange world takes about six months to complete a full orbit around its host star, coming extremely close to the star before shooting off into the distance and then returning in a narrow, oval orbit similar in shape to a cucumber.
“We observed this massive planet making a sharp hairpin turn as it passed close to its star,” study co-author Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy at Penn State, said in a statement. “Transiting planets this eccentric are incredibly rare — and it’s truly incredible that we were able to discover the most eccentric of them all.”
The planet is just 3 million miles (5 million kilometers) from its star, more than 10 times closer to the star than Mercury is to the Sun. For reference, Mercury is at an average distance of about 36 million miles (58 million kilometers) from the Sun, according to NASA.
The extreme orbit causes “huge temperature swings” over the course of the planet’s year, said study co-author Jason Wright, a professor of astronomy and astrophysics at Penn State.
“The temperature at the cloud tops gets high enough to melt titanium during the few days it whizzes past the star,” Wright said via email. “For most of its orbit, it’s farther away, and at its farthest point, the temperature at the cloud tops during the day is comparable to a hot summer day on Earth.”
The research team also discovered that the planet is rotating backwards, or moving in the opposite direction of its star’s rotation – a rare phenomenon that has not been observed on most exoplanets and does not occur in our solar system.
All the peculiarities observed about TIC 241249530 b help astronomers understand how the planet formed.
“While we can’t really go back and observe the process of planetary migration in real time, this exoplanet serves as a kind of snapshot of the migration process,” Gupta said. “Planets like this are hard to find, and we hope it can help us unravel the history of hot Jupiter formation.”
The team ran simulations to determine how the planet came to be in such an unusual orbit and how it might evolve over time. The simulations included modeling the gravitational interactions between TIC 241249530 b and its host star as well as the secondary star.
The research team estimated that the planet likely formed far from its host star and began in a wide, circular orbit similar to Jupiter’s. But the host star’s orbit was misaligned with the second star, which exerted gravitational forces on the planet and elongated its orbit, the researchers said.
“Over many orbits, the gravitational influence of this outer star has altered the orbit of TIC 241249530 b, making it increasingly elongated,” Wright said.
With each pass of the host star, the planet’s orbit loses energy, so astronomers estimate that in hundreds of millions of years the orbit will shrink and stabilize to last only a few days instead of the current 167 days.
Then the planet will become a real hot Jupiter, said Sarah Millholland, study co-author and assistant professor of physics at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology.
“It’s a pretty extreme process in that the changes in the planet’s orbit are massive,” Millholland said. “It’s a big dance of orbits that takes place over billions of years, and the planet just gets caught up in the whirlwind.”
Before TIC 241249530 b, the only other known early hot Jupiter was an exoplanet called HD 80606 b, discovered in 2001. HD 80606 b was considered the planet with the most eccentric orbit until its recent discovery.
HD 80606 has an eccentricity of 0.93 and a shorter orbit of 111 days, and it orbits in the same direction as its star. But otherwise, the planets are practically twins, Wright said. Finding two planets in such a brief stage of planetary orbital evolution is like “stumbling upon a butterfly just as its chrysalis is opening,” he said.
The discovery of a second hot Jupiter precursor helps astronomers confirm the idea that high-mass gas giants transform into hot Jupiters as they migrate from eccentric to circular orbits, the researchers said.
The team hopes to observe TIC 241249530 b with the James Webb Space Telescope to learn about the dynamics of its atmosphere and see how it responds to such rapid warming. And the search continues to find other planets like these that turn into hot Jupiters.
“This system highlights the incredible diversity of exoplanets,” Millholland said. “These are other mysterious worlds that may have wild orbits that tell the story of how they got there and where they’re going. For this planet, its journey isn’t quite over yet.”