April 13, 2024

When stars destroy and eat their own planets

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Credit: NASA/CXC/M.Weiss

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Credit: NASA/CXC/M.Weiss

Our sun is our best friend and our worst enemy. On the one hand, we owe our existence to our star. Earth and the other planets in the solar system formed from the same cloud of gas and dust as the sun.

And without its light, there could be no life on this planet. On the other hand, the day will come when the Sun will wipe out all life on Earth and eventually destroy the Earth itself.

The risks that stars can pose to their planets are highlighted by a new study published in Nature. The authors observed stars similar to our Sun and found that at least one in 12 stars exhibits traces of metals in its atmosphere. They are believed to be scars from planets and asteroids that were ingested by stars.

Planets should never feel too comfortable while orbiting their parent star, as there are at least two ways their star can betray their trust and bring about their violent death.

Tidal disturbance

The first is through a process called “tidal disturbance”. As a planetary system forms, some planets orbit their star along trajectories that are not completely circular or that are slightly tilted with respect to the star’s plane of rotation. When this happens, the gravitational force exerted by the star on the planet will slowly correct the shape or alignment of the wayward planet’s orbit.

In extreme cases, the gravitational force applied by the star will destabilize the planet’s orbit, pulling it closer and closer. If the unfortunate planet gets too close, it will be torn apart by the star’s gravity. This happens because the side of the planet facing the star is a little closer than the opposite side (the difference is the diameter of the planet).

The strength of the gravitational attraction exerted by the star depends on the distance between it and the planet, so the side of the planet facing the star feels a slightly stronger attraction than the opposite side.

On Earth, this difference in the strength of gravity creates the daily ebb and flow of tides. In essence, the Sun is trying to deform the Earth, but it is far enough away that it can only attract the waters of its oceans. But a planet dangerously close to its star will see its crust and core destroyed by these tides.

If the planet is not very close to the star, its shape will only be deformed into the shape of an egg. Just a little closer to the star, and the difference between the gravitational pull on its different sides will be enough to shatter it completely, reducing it once again to a cloud of gas and dust that spirals toward the star and vaporizes in its hellish fires. .

The process of tidal disturbance was first suggested about 50 years ago. Over the past two decades, astronomers—including my group—have observed dozens of bright tidal disruption explosions caused by stars destroyed by supermassive black holes at the centers of galaxies.

Last year, for the first time, a group of astronomers reported observing a similar, weaker explosion, consistent with a planet being disturbed and consumed by its star.

Tidal disruption of planets may be quite common, as shown by the new discovery that at least 1 in 12 stars exhibit signs that they have ingested planetary material.

Other studies have found that between a quarter and a half of all white dwarfs – the remains of stars up to twice the mass of our Sun – bear similar scars. As their name suggests, white dwarfs are incandescent. With surface temperatures of tens of thousands of degrees, the hottest white dwarfs emit ultraviolet light and X-rays with enough energy to vaporize their orbiting planets.

The end of the Earth

Be sure; The Earth will not be destroyed by tidal disturbance. The end of our planet will occur in about five billion years, when the Sun will transition into a red giant.

Stars are powered by the process known as fusion, where two light elements are combined to form a heavier one. All stars begin their lives by fusing the element hydrogen in their cores into the element helium. This fusion process stabilizes them against implosion, due to the incessant pull of gravity, and creates the light that makes them shine. Our Sun has been fusing hydrogen into helium for about 4.5 billion years.

But in 4.5 billion years, the hydrogen in the Sun’s core will run out. All fusion in the core will stop and unopposed gravity will force the star to contract. As the core contracts, it heats up until the temperature is high enough for helium to fuse into carbon.

The merger will once again stabilize the star. In the meantime, however, the star’s outer casings will expand and cool, giving the now-giant star a redder hue. As the red giant Sun expands, it will swallow Mercury, Venus and Earth – and may even reach the orbit of Mars.

Earth may still have another 5 billion years to go, but we won’t be here to witness its extinction. As the Sun burns through its hydrogen reserves, it becomes increasingly brighter: every billion years, its luminosity increases by around 10%.

A billion years from now, the Sun will be bright enough to evaporate Earth’s oceans. So the next time you enjoy the sun’s warm rays, remember: it has everything for us.

More information:
Elizabeth Gibney, Planet-Eating Stars Hide Hidden Chaos in the Milky Way, Nature (2024). DOI: 10.1038/d41586-024-00847-6

Fan Liu et al, At least one in a dozen stars shows evidence of planetary ingestion, Nature (2024). DOI: 10.1038/s41586-024-07091-y

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