Astronomers detect exoplanet with inverted cucumber-shaped orbit

Astronomers have detected a exoplanet with a highly elongated orbit that is experiencing extreme temperature changes — and may be turning into another kind of world.

The exoplanet called TIC 241249530 b orbits a star that is about 1,100 light years from Earth. The body is part of a binary pair, so the planet orbits the primary star, while the primary star orbits a secondary star.

Interactions between the two stars — which have a misaligned orbit — could be responsible for putting this planet on the path to becoming a “hot Jupiter,” researchers reported in a study published Wednesday in Nature magazine.

Astronomers have already found more than 5,600 confirmed exoplanets and 300 to 500 of them are “hot Jupiters”. These planets are massive planet-like gaseous bodies in the Solar System that orbit their host stars closely, heating to scorching temperatures.

While Jupiter takes 4,000 Earth days to complete an orbit around the Sun, “hot Jupiters” complete their journey every few days.

Scientists believe these large planets start out orbiting their stars at a distance, but migrate closer over time. However, they have long questioned how these massive worlds 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-hunting satellite in January 2020, offer rare and revealing insights into what a star on the path to becoming a “hot Jupiter” could be.

“Astronomers have been searching for exoplanets that are likely precursors to ‘hot Jupiters,’ or that are intermediate products of the migration process, for more than two decades, so I was very surprised — and excited — to find one,” said Arvind Gupta, author study principal and NOIRLab postdoctoral researcher who discovered the planet while a doctoral student at Penn State, in a statement.

Observing a changing world

On January 12, 2020, the Transiting Exoplanet Survey Satellite (Tess) collected data suggesting that something was passing in front of the host star TIC 241249530. The telescope monitors the brightness of nearby bodies to look for dips in starlight that could indicate the presence of exoplanets.

Gupta and his colleagues followed the data and determined that a Jupiter-sized star was passing in front of the star. So they took measurements using instruments on the 3.5-meter Wiyn Telescope at the Kitt Peak National Observatory in Arizona in the United States to determine the star’s radial velocity, or how much it wobbles back and forth as it moves. The planet’s gravity pulls it.

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 “eccentric” to refer to the shape of a planet’s trajectory on a scale from zero to one. Zero equals a perfectly circular path. In our Solar System, Earth has an eccentricity of 0.02, while Pluto’s highly oval path around the sun is considered to be 0.25.

TIC 241249530, the newly discovered exoplanet, has an eccentricity of 0.94 — which is more elongated than any other transiting body ever found by astronomers, according to researchers. The peculiar world takes about six months to complete an orbit around its host star, getting extremely close to it before moving away and then returning to a narrow, oval path. similar in shape to a cucumber.

“We caught this massive planet making a sharp turn during its close pass to its star,” said study co-author Suvrath Mahadevan, Verne M. Willaman Professor of Astronomy at Penn State, in a statement. “Transiting planets with such highly eccentric orbits are incredibly rare — and it’s really amazing that we managed to discover the most eccentric one.”

The body is about 4.8 million kilometers from its star, more than 10 times closer to the star than Mercury is to the Sun. For reference, Mercury is located at an average distance of approximately 58 million kilometers from the Sun, so agreement with NASA.

The extreme orbit causes “huge temperature swings” throughout the planet’s year, said study co-author Jason Wright, a professor of astronomy and astrophysics at Penn State.

“The temperature at the top of the clouds gets hot enough to melt titanium during the few days that the planet passes quickly near the star,” Wright said in an email. “For most of its orbit, it is at its most distant and at its furthest point, the heat at the tops of the clouds during the day is like a hot summer day on Earth.”

The research team also discovered that the planet is orbiting backwards, or moving in the opposite direction compared to the rotation of its star — a rare occurrence that has not been seen on most exoplanets and does not happen in our Solar System.

All the peculiarities observed about TIC 241249530 b are helping astronomers understand how it formed.

“While we can’t exactly press the rewind button and watch the planetary migration process in real time, this exoplanet serves as a sort of snapshot of the migration process,” said Gupta. “Bodies like this are hard to find and we hope it can help us unravel the history of the formation of ‘hot Jupiters’.”

“Dance of orbits”

The team ran simulations to determine how the planet could have ended up on such an unusual trajectory, as well as 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.

Astronomers estimated that the planet likely formed far from its host and began in a wide, circular orbit similar to Jupiter. But the host star has a misaligned trajectory with the second, which exerted gravitational forces on the planet and lengthened its path, the researchers said.

“Over the course of many orbits, the gravitational influence of that outer star altered the trajectory of TIC 241249530 b, making it increasingly elongated,” said Wright.

With each pass by the host star, the planet’s path loses energy, so astronomers estimate that, in hundreds of millions of years, the path will slow and stabilize to last just a few days, instead of the 167 days it takes now.

Then the planet will become a true ‘hot Jupiter,’ said study co-author Sarah Millholland, assistant professor of physics at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT).

“It’s a pretty extreme process, as the changes in the planet’s trajectory are massive,” Millholland said. “It’s a big dance of orbits that’s been happening over billions of years, and the planet is just going with the flow.”

Twin “hot Jupiters”

Before TIC 241249530 b, the only other known early-stage “hot Jupiter” was an exoplanet called HD 80606 b, discovered in 2001. This was considered the body with the most eccentric orbit until the recent discovery.

HD 80606 b has an eccentricity of 0.93 and a shorter trajectory of 111 days, orbiting in the same direction as its star. But other than that, the planets are practically twins, Wright said. Finding two bodies at such a brief stage of planetary orbital evolution is like “meeting a butterfly just as its cocoon opens,” he said.

Discovering a second “hot Jupiter” precursor is helping astronomers confirm the idea that high-mass gas giants transform into “hot Jupiters” as they migrate from eccentric to circular orbits, researchers said.

The team hopes to observe TIC 241249530 b with the James Webb Space Telescope to uncover the dynamics of its atmosphere and see how it reacts to such rapid warming. And the search continues for more planets like these that are turning into “hot Jupiters”.

“This system highlights how incredibly diverse exoplanets can be,” Millholland said. “They are mysterious worlds that can have wild orbits that tell a story of how they got to this point and where they are going. For this planet, your journey is not over yet.”

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