This Jupiter-Sized Exoplanet is Unusual for Several Reasons

In a recent study published in the Monthly Notices of the Royal Astronomical Society, a team of international researchers examined exoplanet TOI-4860 b, which is located approximately 80 parsecs (261 light-years) from Earth and has an orbital period of approximately 1.52 days around a low-mass star, or a star smaller than our Sun. Exoplanets orbiting so close to their parent stars aren’t uncommon and commonly known as “hot Jupiters”.

However, TOI-4860 b is unique due its relative size compared to its parent star, along with its lower surface temperatures compared to “hot Jupiters” and possessing large amounts of heavy elements. These attributes are why researchers are classifying TOI-4680 b as a “warm Jupiter”, and could challenge traditional planetary systems formation models while offering new insights into such processes, as well.

“Under the canonical planet formation model, the less mass a star has, the less massive the disc of material is around that star,” said Dr. George Dransfield, who recently submitted her PhD thesis at the University of Birmingham and is a co-author on the study. “Since planets are created from that disc, high-mass planets like Jupiter, were widely expected not to form. However, we were curious about this and wanted to check planetary candidates to see if it was possible. TOI-4860 is our first confirmation and the lowest mass star hosting such a high mass planet.”

For the study, the researchers used a myriad of observatories and instruments, including TRAPPIST-South/North, the SPECULOOS South Observatory, and MuSCAT3, to collect photometric and spectroscopic data on several transits of TOI-4860 b passing in front of its parent star.

In the end, the researchers calculated that TOI-4860 b is approximately 0.76 the radius of Jupiter while its parent star is approximately 0.34 the radius of our Sun. For comparison, Jupiter’s radius is approximately 70,000 kilometers (43,500 miles) while the Sun’s radius is approximately 700,000 kilometers (435,000 miles), making Jupiter’s radius approximately 10 percent of our Sun. Using these numbers, this puts the radius of TOI-4860 b at approximately 53,200 kilometers (33050 miles) with the radius of its parent star at 238,000 kilometers (147,886 miles), making TOI-4860 b’s radius approximately 22 percent of its parent star, or more than double the radius between Jupiter and our Sun. But how did such a large exoplanet come to form around such a small star?

“A hint of what might have happened is hidden in the planetary properties, which appear particularly enriched in heavy elements,” said Dr. Amaury Triaud, who is a professor of exoplanetology at the University of Birmingham, and lead author of the study. “We have detected something similar in the host star too, so it is likely that an abundance of heavy elements catalyzed the planet formation process.”

The researchers determined that TOI-4860 b contains heavy elements based on its density at 1.55 times that of Jupiter, which is comprised primarily of lighter elements such as hydrogen and helium.

Artist illustration of a warm Jupiter gas-giant exoplanet (right) orbiting its parent star, along with several smaller exoplanets. (Credit: Detlev Van Ravenswaay/Science Photo Library)

Based on these findings, specifically pertaining to the large radius of TOI-4860 b compared to its parent star, the researchers refer to TOI-4860 b as a “warm Jupiter” since its surface temperature is approximately 350 degrees Celsius (662 degrees Fahrenheit), far less than “hot Jupiters”, some of which can reach 2750 degrees Celsius (4900 degrees Fahrenheit) on their dayside. Previous studies have identified warm Jupiters as having orbital periods of greater than 10 days, along with their systems hosting additional planets, as well. Thus, TOI-4860 provides astronomers with a unique opportunity to examine these unique exoplanets and gain insights into their formation and evolution.

For next steps, the researchers have been granted time at the Very Large Telescope, in Chile, which they plan to use in hopes of identifying additional warm Jupiters or even exoplanets with similar characteristics.

Examples of other confirmed warm Jupiters include EPIC 211418729 b and EPIC 211442297 b. Both were discovered in 2017 and are located approximately 1,569 and 1,360 light-years from Earth with approximate surface temperatures of 445.85 degrees Celsius (834.5 degrees Fahrenheit) and 408.85 degrees Celsius (768 degrees Fahrenheit), respectively. In 2019, astronomers confirmed the existence of TOI-677 b, which is a Jupiter-sized exoplanet located approximately 463 light-years from Earth with an approximate surface temperature of 979 degrees Celsius (1,794 degrees Fahrenheit).

What new discoveries will astronomers make about warm Jupiters in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

Laurence Tognetti

Laurence Tognetti is a six-year USAF Veteran who earned both a BSc and MSc from the School of Earth and Space Exploration at Arizona State University. Laurence is extremely passionate about outer space and science communication, and is the author of “Outer Solar System Moons: Your Personal 3D Journey”.

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