Habitability Around Bizarre Stars
It was nearly three decades ago that Polish astronomers Aleksander Wolszczan and Dale Frail publicly announced the discovery of the first planets outside of our solar system. The two planets, dubbed Poltergeist and Phobetor, represented a class of planets never observed before, yielding masses larger than that of the Earth, but smaller than Neptune: the super-Earths. A discovery such as this normally would have prompted waves of excitement throughout the scientific community. Instead, these 2000 light-year distant extrasolar newcomers were greeted with an aura of skepticism. This hesitance was partially due to a false claim of another exoplanet announced literally just weeks prior, but perhaps an even more substantial factor was the type of host which the planets were discovered to be orbiting: a pulsar. Immediately, speculations ran abound as to how a pulsar, the compressed dead husk of a massive star, could have retained these small, likely rocky companions through its preceding red giant and supernova phases. Regardless, the planets’ existences were confirmed quickly by means of further observation, and even a third planet, the tiny Moon-sized Draugr, was identified two years later in 1994.
Discovering planets around stars trillions of kilometers away is certainly no cakewalk, but to discover three planets with masses small enough to suggest rocky compositions was a feat more than a decade ahead of its time. Since life as we know it requires a planet with a solid surface for liquid water to pool in, rocky planets outside of our solar system are enticing subjects in the search for extraterrestrial life. Though the pulsar planets of PSR B1257+12 do likely yield Earth-like compositions, the similarities between them and our Pale Blue Dot abruptly end there. Impossible to have survived a supernova, scientists theorize that the three planets (and their host pulsar) were instead the result of a binary white dwarf merger which ejected an accretion disk of material into orbit which slowly coalesced to form the children planets. Charting tight orbits around their 10 kilometer wide, 30,000 degree Kelvin host, the small worlds are baked by heat and doused in deadly levels of UV, x-ray, and gamma radiation. This radiation is intense enough to strip these worlds of volatile molecules, depriving them of any chances of a protective atmosphere or liquid water. Christened “the zombie worlds” by NASA, Poltergeist, Phobetar, and Draugr are probably the last places in the universe that we would expect to find life.
In the search for habitable exoplanets, we have focused our keen eyes to stars similar to our Sun; stellar bodies which are small enough to burn for a long enough time for life to evolve, but not so small as to lock any potentially habitable children planets into a tidally induced death-stare. Though the habitability of smaller, red dwarf stars has been heavily debated in the modern age, there may yet be an entire ocean of overlooked opportunities for life around the outcast stars and stellar objects of our universe. Though these strange and bizarre hosts bear little resemblance to our glowing yellow Sun, they may still boast the conditions necessary for life to evolve and thrive.
Neutron Stars
Neutron stars are extremely hot and compact spheres of atomic material formed in the wake of the deaths of monster stars at least 8 times the mass of our Sun. They can pack the mass of 30 Suns into a volume as small as a city, and can have surface temperatures of hundreds of thousands of degrees Kelvin. To date, we have identified about 2,000 neutron stars, most of which, like PSR B1257+12, are pulsars. Pulsars may not bear conditions conducive to life, but perhaps another brand of neutron star may. In the late 90's, a curious group of seven neutron stars were identified which emit almost no high-energy radiation, instead releasing most of their power in the form of normal thermal energy (i.e. heat). Termed The Magnificent Seven, the discovery of an entire family of “radio-quiet” stellar remnants made it highly unlikely that this growing class of neutron stars were simply pulsars not aligned with the Earth, as was previously believed. Though the mechanisms behind them are yet unknown, perhaps these benign stellar ghosts could serve as bizarre havens for planets with life.
Despite such hosts being extremely hot, their tiny diameters actually make their temperature gradients drop off rather quickly with distance. The result is a very tight habitable zone, spanning a range of just 0.05–0.07 AU; only about 8 times the distance between the Earth and the Moon. Due to the high mass of the host, planets orbiting within this zone would experience an entire year in just 4.4 days, whipping around the neutron star at more than 145 kilometers per second! Though these planets would most certainly be tidally locked, this rapid rotation may actually still be enough to circulate planetary atmospheres, delivering moderate temperatures to the dark side of the world while keeping the light side relatively cool. It may, however, be unlikely that radio-quiet neutron stars have any rocky planets to basque in this habitable zone. Because most neutron stars are known to form from supernovae, any rocky planets orbiting close enough to to be in the subsequent habitable zone would have likely been blown apart before the birth of the neutron star. Furthermore, planets orbiting far enough away to avoid this destruction would not be able to migrate close enough to be in the habitable zone without dispelling enough orbital energy to turn them into balls of molten lava. Still, since not enough is known about the origins of radio-quiet neutron stars, nothing can be ruled out definitively.
White Dwarfs
Neutron stars might not have rocky planets in their habitable zones, but perhaps white dwarfs, their cooler, lightweight counterparts, could have better odds. While neutron stars are formed in the deaths of very massive stars, white dwarfs come to life during the final breaths of all the smaller stars of the galaxy. Because their progenitors end their lives in a much less explosive manner than neutron stars, it may be possible for rocky planets to survive the end of their host star’s life to claim subservience to the new host. The conditions necessary for liquid water around a white dwarf are met at a distance of 0.02 — 0.06 AU, but this distance is not static. Since white dwarfs have no internal mechanism to produce heat, they begin their lives at very high temperatures, and then slowly cool with time. This phenomenon may pose challenges to potentially habitable exoworlds around them.
To determine whether a white dwarf could remain hot long enough for worlds with life to evolve around them, their structure must be examined. White dwarfs are held up against gravity by a property known as electron degeneracy pressure. Because degenerate matter is relatively compressible, the radius of a white dwarf actually shrinks the larger its mass is. This fact is important for potential habitability for two reasons: 1) White dwarfs with more mass can contain more heat energy, and 2) white dwarfs with smaller radii (and thus less surface area) radiate their heat away more slowly. This means that massive white dwarfs with small radii probably represent the best hope for habitability around these stellar remnants. However, the more massive and compact a white dwarf gets, the more its gravitational effects can be felt by orbiting children planets.
Jupiter’s moon, Io, should be a cold, frozen world. With an equilibrium temperature of -163 C (-262 F), it came as a great surprise to astronomers when the Galileo spacecraft sent back images of plumes of smoke and molten rock erupting from Io’s surface in 1995. The culprit for this geologic activity was Jupiter’s large mass, and Io’s tight, non-perfect orbit. The tiny irregularity of Io’s orbit, known as eccentricity, was enough cause the moon to be tugged on by the planet’s powerful gravity in a cyclic pattern during its orbit. This tugging on the moon’s insides maintains it a liquid mantle, and a surface riddled with volcanoes. Because Io does not posses a significant atmosphere, the CO2 released in these eruptions has little effect on it’s surface temperature. However, on a potentially habitable world with an atmosphere more like Earth’s, this effect could be disastrous to life. Planets orbiting within the habitable zones of white dwarfs would need to maintain very small eccentricities (~0.0001; similar to Neptune’s moon Triton) in order to avoid a tidally induced runaway greenhouse effect.
Brown Dwarfs
While white dwarfs and neutron stars are the result of the end of a star’s life, brown dwarfs are the star’s whose lives never even began. Composed largely of compressible hydrogen gas, these worlds may have masses up to 80 times that of Jupiter, despite only boasting radii 15% larger. Astronomers estimate that there may be up to 50 billion brown dwarfs in our galaxy alone, meaning if they have a chance to host life, there are plenty of opportunities for that chance to have come. Yielding masses too small to generate hydrogen fusion like the Sun, brown dwarfs maintain internal heat through either deuterium or lithium fusion, both of which are much less energetic nuclear reactions. This timid glow, however, may yet be enough to provide rocky worlds around them with warmth for liquid water to pool.
Small brown dwarfs do not produce enough heat to place any habitable planets far enough away to not be detrimentally affected by tidal forces. However the largest brown dwarfs, those just on the cusp of of claiming stardom, produce enough internal heat to place a habitable zone at a distance of 0.02–0.03 AU. At this distance, the brown dwarf would appear more than 5 times the diameter of the Sun in our sky, and would glow a deep red or vibrant magenta hue. Like the white dwarf, planets orbiting within this sliver of a habitable zone would still face the threat of runaway tidal effects should their orbits display even small irregularities. Despite being technically stable however, this habitable zone would not last long. Even the largest brown dwarfs exhaust their lithium fuel in a mere half billion years, far shy of the length of time estimated to be necessary for the conditions ripe for life to form. Because their radii are enormous compared to that of a white dwarf or a neutron star, once a brown dwarf’s fuel runs dry, it cools rapidly. A mere 10 million years after its lithium is depleted, the brown dwarf, and any chances for life around it, goes dark.
Black Holes
The destroyers of stars. The keepers of light. The predicted fates of everything in the universe. When considering places where we expect to find life, black holes seemingly reside near the bottom of the list. And yet scientists and astronomers still wonder if life bearing conditions could be met on planets in orbit of such dire objects. As their names suggest, black holes emit no light or heat, and as such, have no predefined habitable zones. Without a source of heat, any orbiting planets would surely freeze over short timescales. Instead, such planets may have access to heat given off by a black hole’s accretion disk: an orbiting ring of gas and dust that has been superheated by intense friction. If the black hole is relatively inactive, this accretion disk may be stable enough to provide billions of years of steady heat to any planets orbiting a safe distance away.
The temperature gradient of a black hole’s accretion disk is a function of both the black hole’s mass, and its rate of mass consumption. For the watery worlds exhibited in the film Interstellar to maintain habitability, Gargantua (the supermassive black hole) would require an incredibly small consumption rate on the order of ~3*10¹³ kg/s; about the mass of a small comet. This rate is roughly 170,000 times less than the supermassive black hole at the center of our galaxy, which is ~20 times less massive. Still, such a tiny accretion rate may be possible if the black hole is in a relatively isolated part of space, and has not consumed a vast amount of stellar material in millions of years. The habitable zone of such a black hole is actually surprisingly friendly to potentially habitable worlds, easily circumventing all tidal locking and heating effects at the expense of experiencing an orbit only once every 14 million years. So long as the black hole has a few Sun’s worth of accretion matter to consume during the planet’s maturation period in the habitable zone, life in its orbit may not be out of the realm of feasibility. One potential hurdle to habitability here is that the emissions of black hole accretion disks peak strongly in the UV spectrum. Any children planets would require thick atmospheres rich in ozone in order to defend against this onslaught of radiation.
