Astronomers have made a groundbreaking discovery of an exoplanet that is in the process of orbiting closer and closer to its evolved host star until the two collide and are destroyed. This is the first time such an event has been observed.

This new discovery provides the first opportunity to study a planetary system at the advanced stage of orbital decay, giving us a deeper understanding of the long-term evolution of planets and their orbits.

It is believed that many exoplanets will eventually meet their demise by being engulfed by their host star, and this process may eventually happen to Earth as the Sun grows older over billions of years. This process is known as “death-by-star.

“We’ve previously detected evidence for exoplanets inspiraling toward their stars, but we have never before seen such a planet around an evolved star,” explains lead author Shreyas Vissapragada. “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 results were published in The Astrophysical Journal Letters today.

The exoplanet that is doomed to be destroyed by its host star is called Kepler-1658b. It was first discovered by the Kepler space telescope, which was launched in 2009 and was known for its ability to search for exoplanets. Interestingly, Kepler-1658b was the first exoplanet candidate that Kepler ever observed, but it took nearly a decade to confirm its existence. It was ultimately added to Kepler’s catalog as the 1658th entry.

Kepler-1658b is an exoplanet similar to Jupiter in terms of size and mass, but it orbits much closer to its host star, earning it the nickname of a “hot Jupiter.” In fact, its orbit is only an eighth of the distance between the Sun and Mercury, the planet that orbits closest to the Sun. Hot Jupiters and other exoplanets that are in such close proximity to their stars are doomed to eventually experience orbital decay, leading to their destruction.

Studying the orbital decay of exoplanets has been difficult for researchers because the process is slow and gradual. In the case of Kepler-1658b, a new study found that its orbital period is decreasing by about 131 milliseconds (thousandths of a second) per year, which is a sign that the planet is moving closer to its star. This decrease in the orbital period is very small, making it challenging to measure.

To detect the gradual decline in Kepler-1658b’s orbital period, researchers needed to observe the exoplanet for multiple years. This was done using three different instruments: the Kepler space telescope, the Palomar Observatory’s Hale Telescope in Southern California, and the Transiting Exoplanet Survey Telescope (TESS), which launched in 2018. These instruments all detected transits, which is when an exoplanet crosses in front of its star and causes a slight dimming in the star’s brightness. Over the past 13 years, the interval between Kepler-1658b’s transits has steadily decreased, indicating that the exoplanet is moving closer to its star.

The reason for the orbital decay experienced by Kepler-1658b is due to tides, which are caused by the gravitational interactions between two orbiting bodies. This is the same phenomenon that leads to the rise and fall of the Earth’s oceans. When two bodies such as Kepler-1658b and its host star interact through their gravity, they distort each other’s shapes and release energy. The amount of energy released and the resulting effect on the bodies’ orbits depend on their distances, sizes, and rotation rates. In the case of Kepler-1658b, the tides are causing it to spiral inward towards its star. On the other hand, the tides between the Earth and the Moon are pushing the Moon further away from our planet.

There is still much that researchers do not fully understand about the dynamics of tides in star-planet systems, so studying the Kepler-1658 system in more detail should provide valuable insights.

The star has reached the stage in its development when it has begun to grow, as our Sun is predicted to do, and has entered what astronomers refer to as a subgiant phase. When compared to stars like our Sun, which haven’t changed much inside, evolved stars should be better at getting rid of the tidal energy that comes from their planets’ orbits. This speeds up the process of an orbit falling apart, which makes it easier to study on human time scales.

The results also help explain why Kepler-1658b seems to be brighter and hotter than expected. This is a strange thing about the planet itself. The team says that the tidal interactions that are making the planet’s orbit smaller may also be giving the planet itself a boost of energy.

Vissapragada says that Jupiter’s moon Io, which has the most volcanoes in the Solar System, is in a similar situation. The pull and push of Jupiter’s gravity on Io melts the inside of the planet. This molten rock then bursts onto the moon’s infamously hellish surface, which is covered with new red lava and yellow sulfur deposits.

Adding more observations of Kepler-1658b should help us learn more about how heavenly bodies interact. And since TESS is going to keep looking at thousands of nearby stars, Vissapragada and his colleagues expect the telescope to find many more exoplanets that circle their host stars’ drains.

“Now that we have evidence of inspiraling of a planet around an evolved star, we can really start to refine our models of tidal physics,” Vissapragada adds. “The Kepler-1658 system can serve as a celestial laboratory in this way for years to come, and with any luck, there will soon be many more of these labs.”

Vissapragada, who just joined the Center for Astrophysics a few months ago and is now working with Mercedes López-Morales as a mentor, is excited to see the science of exoplanets continue to make huge leaps forward.

“Shreyas has been a welcome addition to our team working on characterizing the evolution of exoplanets and their atmospheres,” adds López-Morales, an astronomer at the Center for Astrophysics.

“I can’t wait to see what all of us end up discovering together,” says Vissapragada.

Source: 10.3847/2041-8213/aca47e

Image Credit: Gabriel Perez Diaz/Instituto de Astrofísica de Canarias