The Sun, like other stars, will eventually become a red giant, expanding as far as Mars before its outer material drifts off and its core collapses into a highly dense white dwarf. This may be the eventual fate of the Sun. The remaining planets’ fate is in question. Will their disrupted orbits send them traveling aimlessly through the galaxy, or will they be absorbed by the white dwarf? It is even possible that they could remain undisturbed, orbiting a lifeless star as it gradually cools over trillions of years into a cold, inanimate object.
A new discovery has given us an idea of what might happen to the planets of our Solar System. By using the James Webb Space Telescope, astronomers have captured direct images of two exoplanets that appear to be orbiting white dwarf stars. “If confirmed,” writes astronomer Susan Mullally, “these would be the first directly-imaged planets that are similar in both age and separation to the giant planets in our own Solar System, and they would demonstrate that widely separated giant planets like Jupiter survive stellar evolution.”
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These images are remarkable for several reasons. Usually, exoplanets are not seen directly. Although over 5,500 exoplanets have been confirmed (with thousands more being detected), most of these confirmations are indirect. Astronomers analyze the effects that the exoplanet has on its host star and infer its properties based on those effects. Exoplanets are usually too far away and too small, and any light they produce or reflect is minimal compared to the star’s light; our current technology, while improving, is still unable to detect them.
However, the JWST is the most powerful space telescope ever built, so Mullally and her team used it to search for white dwarf exoplanets. They discovered what appear to be gas giant exoplanets orbiting two different white dwarf stars named WD 1202-232 and WD 2105-82 at distances of 34 and 53 light-years from Earth, respectively. The exoplanets’ exact mass is unclear, but the images place them between 1 and 7 times the mass of Jupiter. More precise masses could be obtained by studying the stars’ radial velocity, which would reveal their subtle wobbles.
What’s more fascinating is the distance between the white dwarfs and their planets. The exoplanet orbiting WD 1202-232 seems to be at a distance of 11.47 astronomical units, just a little farther out than Saturn, which orbits the Sun at 9.5 astronomical units. The exoplanet orbiting WD 2105-82 has an apparent orbital distance of 34.62 astronomical units, similar to Neptune’s orbital distance of 30 astronomical units.
If the exoplanets are the same age as their host stars, it would imply that exoplanets with distances similar to those of the outer planets in the Solar System can survive the violent deaths of their stars and remain in orbit. Only a few white dwarf exoplanets have been discovered to date, which means we don’t have a lot of information to work with when it comes to predicting the fate of the Solar System when the Sun dies. Based on analysis of white dwarf atmospheres, we do know that these dense stellar remnants can swallow up close planets, contaminating their atmospheres with planetary material.
This discovery is yet to be confirmed, as the two objects may be galaxies in the distant background, although the researchers believe that is improbable. This implies that the two candidate worlds are a significant and critical piece of the puzzle regarding what happens to a planetary system like ours when the star reaches the end of its life. The team suggests that follow-up surveys be conducted to identify the nature of their intriguing discovery. “If confirmed using common proper motion, these giant planets will represent the first directly imaged planets that are similar in age, mass, and orbital separation as the giant planets in our own solar system,” they write.
“Future spectroscopy and multi-band imaging of these systems may be possible with JWST, which would improve the observational constraints on the physics and variety of cool giant planet models.”