Kepler’s first exoplanet is spiraling toward its doom

The doomed world appears destined to spiral closer and closer to its expanding star until they collide.


Astronomers have discovered an exoplanet with an aged star in orbit for the first time. The final exoplanet to be found by the Kepler space telescope is destined to spiral ever closer to its expanding star until they smash and obliterate it.

By giving us our first look at a solar system this late in its life cycle, the discovery provides fresh insights into the gradual process of planetary orbital decay. Many worlds, including Earth, are predicted to experience death-by-star in the next 5 billion years. Kepler-1568b is an exoplanet with less than 3 million years left.

The first author Shreyas Vissapragada said, “We’ve previously detected evidence for exoplanets in-spiraling toward their stars, but we have never before seen such a planet around an evolved star.”

“For stars similar to the sun, “evolved” refers to those that have fused all their hydrogen into helium and moved into the next stage of their life. In this case, the star has begun expanding into a subgiant. Theory predicts that evolved stars are very effective at sapping energy from their planets’ orbits, and now we can test those theories with observations.”

The ill-fated exoplanet is known as Kepler-1658b. Its discovery was made possible by the Kepler space telescope, a ground-breaking planet-hunting mission that started in 2009. As the first candidate for a new exoplanet that Kepler ever saw, it was given the name KOI 4.01, or the 4th object of interest discovered by Kepler.

KOI 4.01 was initially dismissed as a false positive. Before scientists learned that the data didn’t fit the model, the scientists believed they were modeling a Neptune-sized object around a sun-sized star; a decade would pass as it observed seismic waves traveling through its star. After scientists demonstrated that the planet and its star are far larger than initially thought, the item was formally added as the 1658th object to Kepler’s catalog.

Kepler-1658b is a so-called hot Jupiter. That distance for Kepler-1658b is only an eighth of the distance between our Sun and Mercury, which has one of its closest orbits. Kepler-1658b orbits its star in just 3.8 days, unlike Mercury’s 88-day orbit.

Kepler-1658b is about 2 billion years old and is in the last 1% of its life. Its star has reached the stage of its stellar life cycle where it has begun to grow, like our Sun is predicted to do, and has entered what astronomers refer to as a subgiant phase. The core structure of evolved stars, as opposed to hydrogen-rich stars like our Sun, should more easily result in the dissipation of tidal energy received from hosted planets’ orbits, according to theoretical predictions. As a result, the orbital decay process would quicken, making examining a timescale relevant to humans simpler.

Orbital decay and collision are unavoidable for hot Jupiters and other planets near their Sun. But because the process is so excruciatingly gradual, monitoring how exoplanets circle down the drains of their host stars has proven difficult. According to the current analysis, Kepler-1658 b’s orbital period is decreasing by 131 milliseconds (thousandths of a second) annually.

Scientists noted, “Detecting this decline required many years of careful observation. The watch started with Kepler and was picked up by the Palomar Observatory’s Hale Telescope in Southern California and, finally, the Transiting Exoplanet Survey Telescope, or TESS, which launched in 2018. All three instruments captured transits, the term for when an exoplanet crosses the face of its star and causes a very slight dimming of the star’s brightness. Over the past 13 years, the interval between Kepler-1658 b’s transits has slightly but steadily decreased.”

“The same phenomenon responsible for the daily rise and fall of Earth’s oceans: tides.”

“The tugging distorts each body’s shape, and energy is released as the planet and star respond to these changes. Depending on the distances between them, their sizes, and their rotation rates, these tidal interactions can result in bodies pushing each other away — the case for the Earth and the slowly outward-spiraling Moon — or inward, as with Kepler-1658b toward its star.”

“Many researchers still do not understand these dynamics, particularly in star-planet scenarios, so the astrophysicists are eager to learn more from the Kepler-1658 system.”

Ashley Chontos, the Henry Norris Russell Postdoctoral Fellow in Astrophysics at Princeton said“Even though physically, this exoplanet’s system is very dissimilar to our solar system — our home — it can still tell us a lot about the efficiency of these tidal dissipation processes and how long these planets can survive.”

Journal Reference:

  1. Shreyas Vissapragada et al. The Possible Tidal Demise of Kepler’s First Planetary System. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/aca47e