Planets swallowed up by stars can “rip off” their shells

(ORDO NEWS) — Scientists have built a model of the absorption of planets by stars during their transformation into red giants and identified several scenarios that differ in mechanisms and observed manifestations, as well as the final state of the system.

In five billion years, the Sun will turn into a red giant and expand tenfold. In doing so, it will engulf Mercury and Venus, and possibly the Earth.

Other stars, with the exception of the smallest and most massive ones, also become red giants, so a similar fate will befall – and has already befallen – many planets throughout the Galaxy.

Some exoplanets orbit their stars much closer than Mercury to the Sun, and are larger than Jupiter, and their absorption should lead to dramatic effects. In addition, astronomers have met red giants enriched in lithium.

This element is rapidly destroyed in the stellar interior, and its presence in the atmosphere of a star of “not the first youth” is an undoubted sign of the recent absorption of a large celestial body by the star.

What happens when a planet is swallowed up by its star? When a star expands into the orbit of a planet, tidal forces increase dramatically.

The planet “raises a wave” behind itself in the matter of the star, the gravity of the wave attracts the planet and slows it down (for a similar reason, Phobos will soon collide with Mars). The planet begins to spiral down towards the star and finds itself inside it.

Surprisingly, the result of this meeting is not always unambiguous. Astronomers have found planets in tight orbits around helium subdwarfs , exposed stellar cores that are still undergoing nuclear fusion.

One example of such a system is Kepler-70 . These planets could not form in their current orbits, and helium subdwarfs are formed due to premature shedding of the shell by a red giant.

Planets swallowed up by stars can rip off their shells 2
The proposed mechanism for the formation of the Kepler-70 planetary system. The planets survive a dive into a red giant, and after scattering, its shells are in very tight orbits

All this means that the planet can not only survive the absorption, but also “tear off” the shell from the giant, after which it will again be “free”. Researchers from the University of California (USA), led by Ricardo Yarza , investigated what happens to a planet inside its star using computer simulations.

Note that the complete absorption model is beyond the scope of even modern computer simulation capabilities. The volume of space affected by the planet is millions of times smaller than the volume of the red giant, so it is now impossible to accurately model the entire system with the same resolution.

However, the scientists were able to build a simplified model that allowed them to isolate several different takeover scenarios.

They limited the full simulation to the region around the planet, and based on the results they calculated the effect of the process on the star. It turned out that absorption does not always pass without a trace for a star, while the planets themselves can survive.

At first, the planet in the model used is considered an indestructible massive sphere and interacts with the star in several ways. First, the planet pushes the gas of the stellar shell out of its way, creating a supersonic shock wave in it.

Secondly, sufficiently massive planets attract the oncoming flow, and this process of gravitational focusing creates a “wake” compression wave – a patch of condensed gas that follows the planet in its orbit inside the star and acts on it with its own gravity. In addition, the planet still “raises” a tidal wave behind it inside the star’s atmosphere.

Planets swallowed up by stars can rip off their shells 1
An image of the planet’s trajectory inside a red giant, and a frame from the computer simulations used. The image size on the left is 100 solar radii and on the right is 0.1 solar radii. The color shows the density of the gas, and the arrows show the direction of its flow. On the right figure, you can see the shock wave created by the planet, the tidal wave (deviation of flows behind the planet upwards), and gravitational focusing (convergence of flows towards the planet’s trajectory)

All these processes slow down the planet. At the same time, its kinetic and orbital energy transforms into thermal energy and is released as an addition to the luminosity of the star.

As the planet sinks, it encounters an increasingly dense environment and increasingly powerful tidal forces, and at some point the assumption of its “indestructibility” ceases to operate.

If drag or tidal forces exceed the planet’s “ultimate strength”, the simulation registers its destruction, and all the remaining kinetic and orbital energy of the planet is immediately transferred to the star.

The larger the star, the lower the energy of gravitational binding of its shell and the easier it is to separate. Note that the described model is conservative – with its help, scientists calculated under what conditions the planet will definitely tear off the shell from the giant, but a more careful consideration of all effects will most likely lower this bar.

The simulation results showed that a planet with a mass greater than 10 Jupiter masses completely rips off the shell from the red giant when immersed in it.

On the contrary, stars that have not yet completed their transformation into a red giant are much more likely to remain intact. A solar-mass star that expands to ten solar diameters can survive a merger even with a red dwarf with a mass of one hundred Jupiter masses (one tenth of the solar mass).

Merging with a planet releases colossal energy. The heaviest planets are significantly slowed down by gravitational focusing and tides, and quickly sink deep into the star, so the energy release is very compressed and intense.

At its peak, which lasts about a year, the luminosity of a star can increase many thousands of times, after which it remains increased several times over for hundreds and thousands of years.

Direct observation of this process is a matter of the near future: the Local Group of galaxies has trillions of stars, and among them there are those that are busy absorbing their planets right now.

Probably, the Vera Rubin Observatory , which will be commissioned in 2023, will be able to find them, and it will greatly increase the possibility of detecting variable and catastrophic phenomena in the Universe.

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