The dead and failed stars known as white dwarfs and brown dwarfs can give off heat that can warm up worlds, but their cooling natures and harsh light make them unlikely to host life, researchers say.
Stars generally burn hydrogen to give off light and heat up nearby worlds.
However, there are other bodies in space that can shine light as well, such as the failed stars known as brown dwarfs and the dead stars known as white dwarfs.
White dwarfs are remnants of normal stars that have burned all the hydrogen in their cores. Still, they can remain hot enough to warm nearby planets for billions of years.
Planets around white dwarfs might include the rocky cores of worlds that were in orbit before the star that became the white dwarf perished; new planets might also emerge from envelopes of gas and dust around white dwarfs.
Brown dwarfs are gaseous bodies that are larger than the heaviest planets but smaller than the lightest stars.
This means they are too low in mass for their cores to squeeze hydrogen with enough pressure to support nuclear fusion like regular stars.
Still, the gravitational energy from their contractions does get converted to heat, meaning they can warm their surroundings.
NASA’s WISE spacecraft and other telescopes have recently discovered hundreds of brown dwarfs, raising the possibility of detecting exoplanets circling them; scientists have already observed protoplanetary disks around a few of them.
White dwarfs and brown dwarfs are bright enough to support habitable zones — regions around them warm enough for planets to sustain liquid water on their surfaces.
As such, worlds orbiting them might be able support alien life as we know it, as there is life virtually everywhere there is water on Earth.
An added benefit of looking for exoplanets around these dwarfs is that they might be easier to detect than ones around regular stars.
These dwarfs are relatively small and faint, meaning any worlds that pass in front of them would dim them more noticeably than planets crossing in front of normal stars.
However, unlike regular stars, white dwarfs and brown dwarfs cool as they age, meaning their habitable zones will move inward over time.
The most obvious peril of a shifting habitable zone is that it could result in a planet getting so cold all the liquid water on its surface freezes solid.
There are other dangers, however — as white dwarfs and brown dwarfs cool, the light they give off would change as well, possibly meaning they would end up sterilizing worlds with dangerous, high-energy radiation.
To be specific, extreme ultraviolet rays would break a planet’s water apart into hydrogen and oxygen. The hydrogen can escape into space, and without hydrogen to bond with oxygen, the world has no water and is not habitable.
Such exoplanets would resemble Venus, with dry atmospheres dominated by carbon dioxide.
In addition, because white dwarfs and brown dwarfs are so dim, their habitable zones already start off very near them.
About one-hundredth the distance between the sun and Earth, which is about one-thirtieth the distance between the sun and Mercury.
White dwarfs should tidally heat planets more than brown dwarfs, since white dwarfs are so massive, the researchers noted.
White dwarfs are only about the size of the Earth, but they are remarkably dense, with masses nearly two-thirds that of the sun.
All in all, the scientists found it unlikely that planets orbiting white dwarfs would ever be truly habitable.
When they are young, white dwarfs would blast planets in their habitable zones with ultraviolet rays that would strip the worlds of water.
When they grow older, their habitable zones would shift closer to them, and the amount of tidal heating might also end up desiccating any planets residing in those zones.
Although the chances for life around white dwarfs and brown dwarfs might look slim, they are not zero, the scientists cautioned.
For instance, a planet might drift into the habitable zone of a white dwarf from a more distant orbit long after the formation of that dead star.
It would still have to contend with tidal heating, but it would have avoided radiation that likely would have sterilized its surface.
More research is needed to understand how planets orbiting white dwarfs and brown dwarfs form, and “particularly the amount of water they form with,” Barnes said, a planetary scientist and astrobiologist at the University of Washington at Seattle
“We also need to understand how the high-energy radiation of brown dwarfs evolves with time. This is the energy that can remove water, but we don’t have any idea how strong it can be, and how long it lasts.”
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