Discovered a planet around a dying star for the first time

The planet is also much larger than the dying star it orbits around.

White dwarfs are actually remnants of sun-like stars that have come to the end of their lives. Our sun will also turn into a white dwarf in about five billion years. Planets orbiting such a dying star are drawn to the star and swallowed up. But apparently some planets managed to escape this dire fate. Because researchers have discovered a planet around a dying star for the first time.

WD 1856 b
The newly discovered planet has been named WD 1856 b and is about the same size as planet Jupiter in our solar system. WD 1856 b is located about 80 light-years from Earth in the vast northern constellation of Dragon. The planet orbits its star every 34 hours; a real white dwarf (see box) of almost 6 billion years old. “The findings are remarkable. “So this planet is about the size of Jupiter but has a very short orbital period,” explains researcher Ian Crossfield. “A year on this planet takes only 1.4 days.” For comparison; the closest planet to our sun – Mercury – takes 90 days to complete a circle around the sun. “WD 1856 b thus rotates very quickly around the white dwarf star.”

More about white dwarfs
As a star ages, the hydrogen fusion in the core of the star stops and swells. Then a red giant arises. When the star then blows away its outer layers of gas and dust, the core contracts, leaving a white dwarf. The star has come to the end of its life. Such a white dwarf is about the size of Earth, but is much more dense (a teaspoon of white dwarf matter weighs about as much as an elephant on Earth!). The dying process also has a major influence on the planets that orbit such a star. Most planets will be drawn to the star and swallowed up.

The discovery of a planet around a dying star is a huge milestone and confirms the existence of these bizarre couples. Usually, while slowly dying, a white dwarf devours the circling planets. But not in the case of WD 1856 b, which somehow managed to escape complete destruction. “This tells us that white dwarfs may have planets, something we didn’t know before,” Crossfield concludes. “There are now people looking for planets around white dwarfs that could potentially be habitable.”

Although white dwarfs no longer perform nuclear fusion, they still give off light and heat as they cool slowly. It’s possible that a planet close enough to a dying star might be in what is called the habitable zone. Planets in this zone can contain liquid water – an important ingredient for life as we know it. “However, this would involve a pretty strange system,” said Crossfield. “Moreover, we have to think carefully about how the planets have survived all this time. But it is a great universe! What we know now is that some types of planets orbit a white dwarf anyway. And that encourages searches for even smaller planets around dying stars. ”

The researchers tracked down WD 1856 b with the help of planet hunter TESS. TESS uses the so-called transit method to detect the exoplanets. In addition, the planet hunter gazes for days on end at fragments of the sky in search of stars whose brightness periodically decreases. Such a ‘dip’ in the brightness of a star may indicate that a planet orbits around the star that occasionally blocks part of the starlight. Yet it is not easy to determine from the data whether it actually concerns a planet. “It only tells you that there is ‘something’ that is blocking the starlight, but not necessarily that it is a planet,” explains Crossfield. “For example, it could also be another faint star instead of a planet.”

To find out whether a planet orbits the white dwarf star or whether it is a passing star, the researchers studied the system with infrared. “The new object could be a small star or a large planet,” says Crossfield. “We were able to find out the difference by studying whether the object emits infrared light. Stars are generally hotter than planets and should glow in the infrared. Planets, on the other hand, are colder than stars. This means that we should only see a little or no infrared light. ” The researchers analyzed the light using the now-defunct Spitzer space telescope. “We didn’t see any infrared light at all,” says Crossfield. In addition, the transits recorded by both TESS and Spitzer were completely consistent. Because of this we were almost certain that we had discovered a planet and not a star. ”

The only question now is how it is possible that WD 1856 b was not swallowed up by the dying star. The team thinks the gaseous planet was drawn to the white dwarf’s gravity long after the star left the red giant phase. Otherwise, the planet would have been crushed by the star in its current orbit. WD 1856 b was probably at first located at a safe distance from the white dwarf, but has drawn closer and closer to the dying star over billions of years, now in a tight, circular orbit around the star.

The big question is, of course, whether the discovery of the first planet orbiting a dying star means that the Earth also has a chance of survival if the sun decays in about five billion years. “That is unlikely,” is the somewhat regrettable answer. But we don’t have to grieve. The research confirms that white dwarfs can indeed harbor planets that may also be in the habitable zone. And that offers a tempting opportunity to search for other life forms. In fact, the unique structure of a white dwarf planet system provides the ideal conditions to open a search for chemical signatures in the planets’ atmospheres. And that way, we can search from afar for traces of life on a planet that no one thought could exist.

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