White dwarfs, the leftover cores of sunlike stars, are incredibly common in the universe. Many of them host planets that may lie within the habitable zones of those stars and may even support life. Now, scientists have outlined how to hunt for that 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 have been able to confirm the existence of only a few thousand, because finding planets in general is pretty hard.
Almost all of the exoplanets we've found orbit stars with masses not too different from the sun's. This is for a few reasons. One, we are searching for planets around sunlike stars because we're interested in finding life like our own. Two, sunlike stars are very common. And three, although smaller red dwarf stars are more common than sunlike stars, they're much dimmer, which makes it harder to find planets.
Related: More than half of all sunlike stars in the Milky Way may have a habitable planet
Over 99.9% of the stars we've found planets around will eventually evolve into white dwarfs, which are Earth-size cores of carbon and oxygen. (This will happen to the sun as well.)
Because sunlike stars are very common, and sunlike stars evolve into white dwarfs, there should also be a lot of planets around white dwarfs. And yet observations there have come up short, with only a handful of exotic examples. One is WD 0806-661b, a gas giant planet nearly eight times more massive than Jupiter that orbits at a distance of over 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 one orbit. Another is PSR B1620-26 (AB) b, a gas giant that orbits a white dwarf-pulsar pair.
There are two challenges for anyone interested in finding exoplanets around white dwarfs. One, they're very small and relatively dim, so the commonly used transit method, in which we stare at a star and wait for the exoplanet to cross in front of it, doesn't work. Two, white dwarfs don't have a lot of standout features in their spectra, so the other popular method, which involves watching the redshift and blueshift of spectral features as an orbiting planet tugs on its parent star, doesn't work either.
Zombie planets rising from the dead
Then there's another challenging question: Is it even possible for planets to survive as their host star dies and becomes a red dwarf? The death of a sunlike star is not pretty. First, as the star consumes planets that orbit too closely, it swells to become a red giant. It then undergoes violent spasms, lasting millions of years, that heave great plumes of material out into the surrounding system and destabilize the other worlds.
But even after all that violence, it might be possible for a white dwarf to end up with planets. Some planets may be far enough away to avoid the carnage, allowing them to cling to their orbits. Interactions between those planets and any newly ejected material from the star can bring those planets closer. Another mechanism is for new planets to form from the wreckage of the old ones, creating a new planetary system once things settle down.
So, theoretically, it's possible to make Earth-like planets around white dwarfs. Because those stars are dim and small, their habitable zones — where the temperatures are just right to allow water to exist as a liquid on a planetary surface — would be very close to the white dwarf itself.
Finding Earth-like planets around white dwarfs would be huge, because it would help us understand the ultimate fate of our own solar system and be a completely new place to look for life in the galaxy.
Living in excess
So how would we search for this alien life? Astronomers have released a road map for hunting for 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 via arXiv.
Because the usual methods of looking for exoplanet transits or shifts in motion won't work for white dwarfs, the astronomers propose a much simpler way to look for planets around white dwarfs: just stare at them. White dwarfs are relatively cool, so any orbiting planet would be relatively warm (especially compared with the ratio of the sun's temperature to Earth's temperature). That means the infrared light coming from a white dwarf would also contain some of the infrared light from the orbiting planet. By comparing that combined light with a white dwarf that we know doesn't have any planets around it, we could detect the exoplanet.
The astronomers found that the Webb telescope could look at the nearest 15 white dwarfs and potentially find planets in their habitable zones. But this technique will work only if the planet has the right size and temperature. For example, this method will be able to find an Earth-like planet that's warmed by greenhouse gases (as our planet is) or a smaller planet that's much hotter. If the planet is too small or too cold, then its light won't show up to a detectable level in the combined infrared light from the system.
What's more, if that exoplanet has substantial amounts of carbon dioxide, this method will be able to pick that out, too. While finding that molecule won't be a sure sign of life, it would be an encouraging finding worthy of follow-up observations.
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