WIYN’s 3.5-meter telescope discovers exoplanet with odd orbit

Using the WIYN 3.5-meter telescope at the National Science Foundation’s Kitt Peak National Observatory, a program of NSF NOIRLab, astronomers have discovered the extreme orbit of an exoplanet on its way to becoming a hot Jupiter. Not only does this exoplanet follow one of the most dramatically elongated orbits of any known exoplanet, it also orbits its star backwards, providing insight into the mystery of how hot Jupiters evolve.

There are currently more than 5,600 confirmed exoplanets in just over 4,000 star systems. Within this group, about 300-500 exoplanets fall into the exotic category known as hot Jupiters — large Jupiter-like exoplanets that orbit close to their stars, some as close as Mercury to our Sun. How hot Jupiters end up in such close orbits is a mystery, but astronomers hypothesize that they start out far from their star and then migrate inward over time. The early stages of this process have rarely been observed, but with this new analysis of an exoplanet with an unusual orbit, astronomers are one step closer to unraveling the mystery of hot Jupiters.

The discovery of this exoplanet, called TIC 241249530 b, was made possible by NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020, when it detected a dip in the star’s brightness consistent with a single Jupiter-sized planet orbiting or passing the star. To confirm the nature of these fluctuations and rule out other possible causes, a team of astronomers used two instruments on the 3.5-meter WIYN telescope at the National Science Foundation’s Kitt Peak National Observatory (KPNO), a program of the NSF NOIRLab.

The team first used the NASA-funded NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) in a technique that helps “freeze” the atmospheric flash and eliminate any extraneous sources that could confuse the signal’s source. Then, using the NASA-funded NEID spectrograph, the team measured TIC 241249530 b’s radial velocity by carefully observing how its host star’s spectrum, or wavelengths of emitted light, shifts as the exoplanet orbits it.

If this exoplanet is part of our solar system
Its orbit would look like this – Image: NOIRLab/NSF/AURA/R. Proctor

Arvind Gupta, a postdoctoral researcher at NOIRLab and lead author of the paper published in Nature, credited NESSI and NEID as crucial to the team’s efforts to characterize and confirm the exoplanet signal. “NESSI gave us a much sharper view of the star than would otherwise be possible, and NEID precisely measured the star’s spectrum to detect the shifts caused by the exoplanet,” Gupta explained. In particular, Gupta noted the unique flexibility of NEID’s observing schedule, which allows the team to quickly adapt its observing plan to new data.

“WIYN plays a critical role in understanding why planets in other solar systems vary so much from one system to another,” said Chris Davis, NSF NOIRLab program manager. “The NSF-NASA collaboration on the NN-EXPLORE program continues to yield exciting results in exoplanet research.”

A detailed analysis of the spectrum confirmed that the exoplanet is about five times the mass of Jupiter. The spectrum also revealed that the exoplanet follows a highly eccentric, or elongated, orbit. The eccentricity of a planet’s orbit is measured on a scale from 0 to 1, with 0 representing a perfectly circular orbit and 1 representing a highly elliptical one. This exoplanet has an orbital eccentricity of 0.94, making it more eccentric than all other exoplanets found using the transit method. [1]By comparison, Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25; Earth’s is 0.02.

If this planet were part of our solar system, its orbit would stretch from its closest approach, which is 10 times closer to the sun than Mercury, to its farthest point. This extreme orbit would heat the planet from a summer day to temperatures hot enough to melt titanium. To make the exoplanet’s orbit even more bizarre, the team also discovered that it orbits backwards, in the opposite direction to its host star’s rotation. This is something astronomers don’t see in most other exoplanets, or in our solar system, and it helps the team explain the exoplanet’s history.

The exoplanet’s unique orbital characteristics also provide an indication of its future orbit. The initial eccentric orbit and extremely close proximity to the host star are expected to shift the planet’s orbit, with tidal forces on the planet pulling energy from the orbit and causing it to gradually shrink and envelop it. Discovering this exoplanet before this migration occurs is valuable because it provides important insight into how hot Jupiters form, stabilize, and evolve over time.

“Although we can’t time-lapse and watch the planet migrate in real time, this exoplanet is a kind of snapshot of the migration process,” Gupta said. “Planetary objects like this are extremely rare and difficult to find, and we hope they will help us unravel the story of how hot Jupiters formed.”

“We’re particularly interested in what we can learn about the dynamics of this planet’s atmosphere as it passes through one of its hot paths toward its star,” said Jason Wright, a Penn State professor of astronomy and astrophysics who led the project when Gupta was a researcher. “Telescopes like NASA’s James Webb Space Telescope have the sensitivity to detect changes in the atmosphere of this newly discovered exoplanet as it undergoes rapid heating, so the team can learn more about the exoplanet.”

TIC 241249530 b is only the second exoplanet ever discovered that exhibits a pre-migration phase of hot Jupiters. Together, these two examples support the idea that higher-mass gas giants evolve into hot Jupiters as they migrate from highly eccentric orbits to narrower, more circular ones. “Astronomers have been searching for more than two decades for exoplanets that are likely precursors to hot Jupiters, or intermediate products of the migration process, so I was very surprised—and excited—to find one,” Gupta said. “This is exactly what I was hoping to find.”

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[1] One exoplanet with a higher eccentricity has been found. HD 20782 b has an eccentricity of 0.956 but is not in transit, so its orbital orientation relative to its host star cannot be determined. This highlights the importance of the discovery of TIC 241249530 b, whose orbital properties can be determined thanks to a transit to its star.

source: NOIRLab