White dwarfs, the remnant cores of sun-like stars, are incredibly common in the universe. Many of them host planets that may be in the habitable zones of these stars and may even harbor life. Now scientists have explained how to hunt this possible life.
Most estimates place the total number of planets in the Milky Way somewhere between a few hundred billion and a trillion. That’s right – a trillion. However, astronomers could only confirm the existence of a few thousand, since finding planets in general is quite difficult.
Almost all the exoplanets we found orbiting stars with masses not too different from those of the sun. This is for several reasons. First, we search for planets around solar stars because we want to find life like our own. Two, solar stars are very common. And three, although smaller red dwarf stars are more common than solar stars, they are much dimmer, making it more difficult to find planets.
Related: More than half of all sun-like stars in the Milky Way could have a habitable planet
More than 99.9% of the stars around which we have found planets will eventually evolve into white dwarfs, which are Earth-sized nuclei of carbon and oxygen. (This will arrive in the sun as well.)
Because sun-like stars are very common, and like the sun stars evolve into white dwarfs, there should also be a lot of planets around the white dwarfs. Yet sightings there have been scant, with only a handful of exotic examples. One is WD 0806-661b, a gas giant planet nearly eight times more massive than Jupiter which orbits a distance of more than 2,500 astronomical units, or 232.5 billion miles (373.7 billion kilometers), from its white dwarf star, meaning it takes more than 158,840 Earth years to complete an orbit. Another is PSR B1620-26(AB)b, a The gas giant which orbits a white dwarf-pulsar pair.
There are two challenges for anyone wishing to find exoplanets around white dwarfs. First, they are very small and relatively dim, so the commonly used transit method, in which we look at a star and wait for the exoplanet to pass in front of it, does not work. Two, white dwarfs don’t have many remarkable features in their spectra, so the other popular method, which is to look at the redshift and blueshift spectral characteristics when an orbiting planet shoots at its parent star, doesn’t work either.
Zombie planets rising from the dead
Then there’s another tough question: is it even possible for planets to survive as their host star dies and becomes a red dwarf? The death of a sun-like star is not pretty. First, as the star consumes planets that orbit too closely, it swells into a red giant. It then undergoes violent spasms, lasting millions of years, which raise great plumes of matter in the surrounding system and destabilize the other worlds.
But even after all this violence, it might be possible for a white dwarf to end up with planets. Some planets may be far enough away to avoid carnage, allowing them to cling to their orbits. Interactions between these planets and any newly ejected material from the star can bring these planets closer together. Another mechanism is that new planets form from the wreckage of old ones, creating a new planetary system once things settle down.
So, theoretically, it is possible to do Earth-like planets around white dwarfs. Because these stars are faint and small, their habitable zones — where temperatures are just right for water to exist in liquid form on a planetary surface — would be very close to the white dwarf itself.
Finding Earth-like planets around white dwarfs would be huge, as it would help us understand the ultimate fate of our own solar system and be a completely new place to search for life in the galaxy.
living in excess
So how could we search for this extraterrestrial life? Astronomers have released a roadmap for hunting exoplanets around white dwarfs using the James Webb Space Telescope. They detailed their plans in a paper accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, and the preprint is available through arXiv.
Because the usual methods of finding transits or changes in exoplanet motion won’t work for white dwarfs, astronomers come up with a much simpler way to find planets around white dwarfs: just look at them. White dwarfs are relatively cool, so any orbiting planet would be relatively hot (especially compared to the ratio of the Sunthe temperature at Earthtemperature). This means that the infrared light coming from a white dwarf would also contain some of the infrared light from the orbiting planet. By comparing this combined light with a white dwarf that we know has no planets around it, we were able to detect the exoplanet.
Astronomers have found that the Webb Telescope can observe the 15 nearest white dwarfs and potentially find planets in their habitable zones. But this technique will only work if the planet is the right size and temperature. For example, this method will be able to find an Earth-like planet that is warmed by greenhouse gases (like our planet) or a smaller planet that is much hotter. If the planet is too small or too cold, its light will not appear at a detectable level in the combined infrared light of the system.
In addition, if this exoplanet contains substantial amounts of carbon dioxide, this method will also be able to detect it. Although the discovery of this molecule is not a sure sign of life, it would be an encouraging discovery worthy of follow-up observations.
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