How do planets survive being torn apart by dying stars?

Planets revolve around stars. This is obvious to most people even with a passing interest in astronomy!

But, as we know, nothing lasts forever. Eventually the stars run out of fuel, and some stars end their lives by expanding and engulfing the planets they orbit.

You can learn more about how this affects our solar system in our articles What will happen when our sun dies? And What will happen to our solar system in the future?

A star like our sun swells up into a red giant and swallows up the planets orbiting around it.  Credit: Stocktrek Images/Tomasz Dabrowski/Getty Images

Illustration of a star like our Sun bulging into a red giant and engulfing the planets orbiting it. Credit: Stocktrek Images/Tomasz Dabrowski/Getty Images

But can some planets survive this process? If so, how?

Dimitri Veras Associate Professor and STFC Ernest Rutherford Fellow at the University of Warwick, specializing in extrasolar fate asteroidsAnd cometsMoons and planets.

We spoke to Professor Veras to learn more about the process by which dying stars destroy planets, and how some manage to avoid annihilation.

Dimitri Ferras is Associate Professor and STFC Ernest Rutherford Fellow at the University of Warwick.

Dimitri Ferras is Associate Professor and STFC Ernest Rutherford Fellow at the University of Warwick.

How are planets destroyed at the end of a star’s life?

In the process of becoming a white dwarf, a star like our sun undergoes major changes related to its size and brightness, when the star turns into a so-called “giant star”.

The star increases in size by a factor of hundreds, and it will easily swallow very close planets.

For example, in the solar system, Mercury and Venus would be swallowed up.

Some distant planets will be gravitationally disturbed in close orbit around the star.

We have some examples of giant planets perturbed in this way, but no terrestrial planets yet.

Immediately after the giant stage, the star shrinks into a small, compact white dwarf.

An artist's impression of a white dwarf star.  Credit: 7activestudio/Getty Images

An artist’s impression of a white dwarf star. credit:
7activestudio/Getty Images

Can some planets survive this?

Our investigation was one of the first-ever specialized studies of tidal effects between white dwarfs, rocky planets like Earth and Mars, and asteroids.

Tidal forces occur because gravity acts more strongly on one side of the planet than on the other.

This difference stretches the planet and the stretch becomes more intense the closer the planet is to the star.

Hence, there is a critical distance at which the planet can no longer survive.

What did your investigation include?

We created a procedure for calculating these tidal effects and provided a kind of “survival guide” for planets with different properties such as distance, viscosity, and rotation.

This allows us to determine if a given planet will survive or be disintegrated by the white dwarf, and if so, when.

We then used numerical simulations to implement this theory and provide specific examples.

Artist's impression of a Jupiter-like exoplanet orbiting a white dwarf.  Credit: WM Keck Observatory/Adam Makarenko

Artist’s impression of a Jupiter-like exoplanet orbiting a white dwarf. Credit: WM Keck Observatory/Adam Makarenko

Are small, rocky planets around white dwarfs particularly at risk?

Tidal force is a function of many variables.

One is the size of the planet, because the gradient of gravitational forces is greater in larger planets, and creates a strong and fast internal gravitational force toward the star.

Hence, the smaller planets have a higher chance of surviving because they are not moving towards the star as quickly.

Another variable that affects tides is how “hard” a planet is, which is measured by its internal viscosity.

The more viscous a planet is, the more it will be able to resist gravity towards the star and destruction.

What other factors affect the survival of the planet?

One of the most important is the distance from the white dwarf.

Another weaker but not negligible factor is the rotation rate of both the planet and the star.

Planets can be destroyed around many stars, not just white dwarfs.

Unlike other stars, white dwarfs offer a glimpse into the future of the solar system and the future of almost all known exoplanets.

What will happen when our sun dies?  This is an image of our host star taken by NASA's Solar Dynamics Observatory.  Credit: Solar Dynamics Observatory, NASA.

An image of the Sun taken by NASA’s Solar Dynamics Observatory. Credit: Solar Dynamics Observatory, NASA.

What is the fate of the surviving planet when its white dwarf star radiates all of its energy?

The radiation of a white dwarf has no direct effect on the gravitational tide.

However, faded white dwarfs are very old, and the time scale over which gravitational tides operate is a major determinant of their survival.

For example, a faint five-billion-year-old white dwarf is less likely to host a surviving planet than a faint three-billion-year-old white dwarf, simply because in the former case there would have been more time for the gravitational tide to pull the planet in and destroy it.

What should astronomers look for when studying exoplanets?

One of the next major discoveries will be an Earth-like planet orbiting a white dwarf.

So far, only giant planets and small asteroids have been found orbiting these stars.

We have shown that Earth-like planets can remain stable around white dwarfs for long periods.

Our results provide a “proof” and incentive for observers to finally find them, because we’ve shown that in many cases they can survive for at least hundreds of millions of years.

This interview originally appeared in the January 2023 issue of BBC Sky at Night Magazine.

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