“Hot Jupiter” is in a possible death spiral

A distant planet is in a death spiral and is about to be engulfed by its parent star.

Kepler-1658b is the first inspiring planet discovered around an “evolved” star, a star that has left its original life. The star – Kepler-1658 – is about 1.5 times the mass of our Sun and expanded to almost 3 times the diameter of the Sun in its later stages of life, earning it the designation of sub- giantess.

If Kepler-1658b maintains its current trajectory, it will meet its fate in about 2.5 million years.

However, as the complicated discovery of the planet and its star has shown, nothing is certain. “It’s a very confusing system,” said Ashley Chontospostdoctoral fellow at Princeton University and member of the team that discovered the planet’s shrunken orbit.

Kepler-1658b was the first exoplanet discovered by the Kepler Space Telescopewho found thousands of bodies in his lifetime using the transit techniques. The telescope measured tiny dips in a star’s brightness as a planet passed in front of it.

Kepler gazes into a galaxy filled with his exoplanet discoveries in this illustration commissioned for the Space Telescope Retirement. Credit: Nasa

Early in its mission, Kepler recorded such lows since Kepler-1658. However, astronomers had originally cataloged the star as belonging to the main sequence – stars like the Sun that still burn hydrogen in their cores. The researchers expected the star to be much smaller than it is, so the initial transit signals “didn’t make sense”, said Shreyas Vissapragadapostdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics and lead author of the new study. The transit indicated a planet roughly the size of Neptune, the third largest planet in our solar system. However, the system also produced a secondary eclipse when the planet passed behind the star. At Kepler’s distance of 1658, a planet the size of Neptune would not be bright enough to see, so there would be no evidence of the secondary eclipse.

Kepler-1658b was dismissed as a false positive and forgotten.

“Suddenly a nearby hot Jupiter made sense.”

That is, until Chontos began to study vibrations on the surface of stars in the Kepler catalog. Because the telescope kept a constant eye on the stars in its field of view, recording brightness levels every half hour or less, it detected “jolts” caused by sound waves reverberating through the stars. Assemble vibrations – a technique known as asteroseismology– revealed details about the interiors of the stars.

In the case of Kepler-1658, they showed that the star was much further along in life than expected and therefore about 3 times larger. This meant that the transiting planet was also 3 times larger, making it large enough and bright enough to contribute to the overall luminosity of the system when not eclipsed by the star. “Suddenly a nearby hot Jupiter made sense,” Chontos said. “This discovery was completely accidental.

A Hot Jupiter is a massive planet comparable to Jupiter, the giant of our own solar system, which orbits so close to its star that it is extremely hot. In this case, Kepler-1658b is roughly the size of Jupiter, but with almost 6 times its mass. “Even the combined masses of all the planets of [our] the solar system doesn’t fit that,” Chontos said. The planet orbits its star once every 3.85 Earth days, compared to 88 days for Mercury, the closest planet to the Sun.

Change a planetary clock

“The clock had changed – transits were occurring noticeably earlier than expected.”

Kepler observed the system for about 4 years, so he got a pretty good, but not perfect, measurement of the orbital period. This seemed to show that Kepler-1658b was following a regular path around the star.

At the same time, Chontos was studying the vibrations of the system, however, Vissapragada was conducting his own observations. (One night, in fact, he and Chontos ran into each other while racing the 200-inch Hale Telescope at the Palomar Observatory, where both were examining the system.)

Vissapragada obtained data from two Hale sessions plus three series of one-month observations by the Transiting exoplanet study satellite (TESS), a space telescope designed to discover and study exoplanets. Combined with earlier Kepler data, the data provided a 13-year base of observations.

“They showed that the clock had changed – transits were happening noticeably earlier than expected,” Vissapragada said. Kepler-1658b’s orbital period was shrinking by 131 milliseconds per year (plus or minus about 20 milliseconds), suggesting the planet will spiral into the star in about 2.5 million years.

The shrinking of the orbit is likely the result of tidal effects. “We think we know the total energy in the system,” Chontos said. “The planet deposits energy into the star, causing it to spin faster and the planet’s orbit to shrink.” A small amount of the system’s total energy could also be dissipated within the planet, explaining some minor oddities in its orbit, Vissapragada added.

Eliminate alternatives

However, inspiration is not the only possible explanation for the apparent change in orbital period. The timing might seem to change if the system was heading our way, for example. By measuring the system’s radial velocity – its movement towards or away from us – the team ruled out this possibility. It also ruled out the possibility that we were only seeing part of the orbit’s precession period – a “wobble” in the orbit. “We believe we have ruled out all other probable causes,” Vissapragada said.

“The evidence for the inspiration of the planets is plausible, and this paper makes a good case for this being the case for this planet,” said Girish Duvvuri, a graduate research assistant at the University of Colorado at Boulder who has studied the disappearance of exoplanets but was not involved in this project. “While I can’t say they’ve exhausted all alternative hypotheses, they’ve covered everything I can think of.”

Even so, no one can say Kepler-1658b’s fate is sealed. The process of orbital evolution of planets around evolving stars is poorly understood, so several outcomes are possible.

“The whole dissipation process is very complicated,” Chontos said. “It involves the obliquity, the eccentricity, the distance – all these different aspects of the orbit that can change over time. “It won’t circularize and its migration will stop, just stop. At some point, the planet might even migrate outward. But for now, that’s just speculation.”

Astronomers hope to narrow the possibilities with additional observations of the system by TESS and other ground and space telescopes. And they said finding similar systems would also help.

“We need to examine more of these systems to determine exactly how this evolution works,” Vissapragada said. “TESS should give us many more examples over the next decade, so we’ll have a fairly large sample to see if this mechanism is common.”

—Damond Benningfield, science writer

Quote: Benningfield, D. (2023), “Hot Jupiter” is in a possible death spiral, Eos, 104, https://doi.org/10.1029/2023EO230028. Published on January 31, 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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