Astronomers have discovered for the first time an exoplanet with an ancient star orbiting Earth. The last exoplanet to be found by the Kepler space telescope is destined to get closer and closer to its expanding star until they break it apart and destroy it.
By giving us our first look at a solar system this late in its life cycle, the discovery offers new insights into the gradual process of planetary orbital decay. Many worlds, including Earth, are predicted to die star-by-star over the next 5 billion years. Kepler-1568b is an exoplanet with less than 3 million years to go.
First author Shreyas Vissapragada said: “We’ve found evidence of exoplanets spiraling toward their stars before, but we’ve never seen such a planet around an evolved star.”
“For stars similar to the sun, ‘evolved’ refers to stars that have fused all their hydrogen into helium and moved on to the next stage of their lives. In this case, the star has begun to expand into a subgiant. Theory predicts that evolved stars are very effective at extracting energy from the orbits of their planets, and now we can test those theories with observations.”
The ill-fated exoplanet is known as Kepler-1658b. The discovery was made possible by the Kepler Space Telescope, a pioneering planet-hunting mission that began in 2009. As the first new exoplanet candidate Kepler ever sighted, it was named KOI 4.01, or the 4th object of interest discovered by Kepler.
KOI 4.01 was initially dismissed as a false positive. Before discovering that the data didn’t fit the model, the scientists thought they were modeling a Neptune-sized object around a sun-sized star; a decade would pass as it watched seismic waves travel through its star. After scientists showed that the planet and its star are much larger than originally 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 one 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, in contrast to 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 begins to grow, as our sun is predicted to do, and has entered what astronomers call a subgiant phase. The core structure of evolved stars, unlike hydrogen-rich stars like our Sun, should more readily accommodate the dissipation of tidal energy received from the orbits of hosted planets, according to theoretical predictions. As a result, the orbital decay process would accelerate, making exploration of a time scale relevant to humans easier.
Orbital decay and collision are inevitable for hot Jupiters and other planets near their sun. But because the process is so painfully gradual, it has proven difficult to monitor how exoplanets orbit through the sewage system of their host stars. According to current analysis, Kepler-1658 b’s orbital period is decreasing by 131 milliseconds (thousandths of a second) annually.
Scientists noted, “Detecting this decline took many years of careful observation. The watch started with Kepler and was picked up by the Hale Telescope at the Palomar Observatory 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 eclipse of the star’s brightness. Over the past 13 years, the interval between transits of Kepler-1658 b has decreased slightly but steadily.”
“The same phenomenon responsible for the daily rise and fall of Earth’s oceans: tides.”
“The pulling distorts the shape of each body and energy is released as the planet and star respond to these changes. Depending on their distances, their sizes and their rotational speeds, these tidal interactions can cause bodies to push each other away — the case for Earth and the slowly outward-rotating moon — or inward, as Kepler-1658b toward its star.”
“Many researchers still don’t understand these dynamics, especially in star-planet scenarios, so the astrophysicists are eager to learn more about the Kepler-1658 system.”
Ashley Chontos, the Henry Norris Russell Postdoctoral Fellow in Astrophysics at Princeton said: “Although this exoplanet’s system is physically very different from 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.”
- Shreyas Vissapragada et al. The possible demise of Kepler’s first planetary system. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/aca47e