How some planets can survive their stars’ deaths
When a star dies, which of its planets have the best chance of surviving? It turns out that the smallest and densest rocky worlds would be the most likely to escape a crushing, fiery fate. This was the conclusion of a new study by astrophysicists at the University of Warwick in the U.K., who published their findings in the peer-reviewed Monthly Notices of the Royal Astronomical Society on May 1, 2019.
The scientists describe their research as a “survival guide for exoplanets” that outlines how different kinds of planets would fare when their host star dies and turns first into a red giant and then a white dwarf, the hot, burnt-out core of the once-active star. Massive-enough stars would ultimately explode as supernovae, blasting their outer layers into space. As might be expected, many planets would be destroyed during the transition from ordinary star to white dwarf, but some would be able to escape their doom, depending on various factors. According to the new study’s lead author Dimitri Veras:
The paper is one of the first-ever dedicated studies investigating tidal effects [gravitational effects] between white dwarfs and planets. This type of modeling will have increasing relevance in upcoming years, when additional rocky bodies are likely to be discovered close to white dwarfs.
So which planets would be the most likely to be obliterated?
According to these astronomers’ calculations, the most vulnerable planets would be those that get moved into a star’s “destruction radius,” the distance from the star where an object held together only by its own gravity can disintegrate due to tidal forces. Tidal forces are gravitational forces; they stretch a body towards and away from the center of mass of another body. Stretch a planet enough, and the whole world will disintegrate. Planets in orbit around a star would become subject to shifts in tidal forces as the star collapses to the white dwarf stage and ultimately turns into an extremely dense relic of its former self. Those forces could also move planets into entirely new orbits, with some being pulled in closer to the star, but others pushed outward.
More massive planets have a greater chance of being destroyed than less massive ones, these astronomers found. However, for smaller planets, a key factor seems to be viscosity: the ease or resistance to flow in a planet’s bodily makeup. Saturn’s moon Enceladus – with its subsurface ocean and outer ice crust – is an example of a smaller low-viscosity body. The new study shows that even Earth-sized, low-viscosity planets could easily be swallowed up by the dying star.
High-viscosity exo-Earths, with dense cores, are a different story. They’d have a better chance at survival, as they would be swallowed by the star only if they reside at distances within twice the separation between the center of the white dwarf and its destruction radius. But those kinds of planets are more difficult to calculate the survival potential for. As Veras explained:
Our study, while sophisticated in several respects, only treats homogenous rocky planets that are consistent in their structure throughout. A multi-layer planet, like Earth, would be significantly more complicated to calculate but we are investigating the feasibility of doing so too.
So the mass of a planet and its distance from the star are crucial factors in whether it can survive a star’s violent transition to a white dwarf. But there is always a safe distance, as well. Generally speaking, a dense, rocky and homogenous planet which resides at a location from the white dwarf that is more than about one-third of the distance between Mercury and the sun is guaranteed to avoid being swallowed from tidal forces as the star collapses.
Knowing what kinds of planets could survive, and their probable locations, can help astronomers search for planets that still exist around white dwarf stars. As Veras said:
Our study prompts astronomers to look for rocky planets close to – but just outside of – the destruction radius of the white dwarf. So far observations have focused on this inner region, but our study demonstrates that rocky planets can survive tidal interactions with the white dwarf in a way which pushes the planets slightly outward.
Interestingly, the very first exoplanets ever discovered were found orbiting a pulsar, an even more extreme type of dead star called a neutron star (the collapsed core of a star that underwent a supernova explosion). Three exoplanets were found orbiting the pulsar PSR B1257+12 (previously called PSR 1257+12), in 1992 and 1994, and were named PSR 1257+12 A, B, and C. Astronomers were surprised, since at the time it was only thought that main sequence stars could host planets.
PSR B1257+12 is a kind of neutron star, with a rotation period of 6.22 milliseconds (9,650 rpm). It has an estimated mass of 1.4 million Earths but is very tiny, only about six miles (10 km) across. It formed when a white dwarf transformed into a rapidly spinning neutron star during the process of two white dwarfs merging with each other. Pulsar planets seem to be a lot rarer than white dwarf planets however, with only four confirmed so far.
Finding and studying more exoplanets that still orbit white dwarf stars will also help scientists better understand what will likely happen to Earth billions of years from now, when our own sun reaches the end of its life and transforms into a white dwarf. Another related recent study, also from the University of Warwick, provided the first evidence that white dwarf stars eventually crystallize, becoming crystal white dwarfs. This includes our own sun after it turns into a white dwarf, about 10 billion years from now.
Bottom line: Scientists now have a better idea of what kinds of planets are the most likely to survive the death of their host star, as it condenses into a white dwarf. It all comes down to mostly mass and distance, and the new study also provides clues as to what kind of fate awaits our own Earth after our own sun dies.