The Undiscovered Country: Planets of Dead Stars
The solar system is dying. It’s happening slowly, but inevitably. In approximately six billion years, the Sun will become a red giant: a bloated star burning up hydrogen in its outer shell. It will have engulfed Mercury and Venus and, possibly, Earth. While our beloved planet may survive the event, it would do so as a scorched rock, uninhabitable long before that celestial event because of the increasing luminosity of the Sun and temperatures on the planet. Is it the end of habitability in our solar system? Not quite.
Some icy moons and possibly even faraway dwarf planets possess water oceans hidden beneath their icy crusts. During the red giant stage, some of the moons of Jupiter and Saturn may transition to open water oceans under a vapor atmosphere, but it may be a short-lived state, since they could quickly lose their water to space. If we’re being optimistic, the new habitable zone wouldn’t stay around the orbits of the giants for longer than a few hundred million years, and it would eventually reach beyond the Kuiper Belt and melt its numerous icy worlds.
As the hydrogen in the shell runs out, the dying star ignites helium in its core, and its luminosity decreases for some time, moving the habitable zone back to the giant planets and their moons. But this source of energy won’t last long as there would then be a few brief flashes as the helium in the star’s outer shell ignites. The result could feature the Sun at its brightest, increasing its luminosity a thousandfold but only for years to maximum hundreds of years each time. What a sight it would be—but best observed from a very large distance. Earth, if it has survived, has moved to a more outward orbit and become a hot barren rock. By now, the habitable zone has once again shifted to the Kuiper Belt and further, enabling Pluto, Eris, Sedna, and many other water- and organics-rich dwarf planets to bask in balmy temperatures for mere millions of years.
Planets in systems of lower-mass stars may be luckier. Imagine a large moon of an exo-Jupiter around a less massive star that still undergoes the red giant phase. It may remain in the new, “post-main sequence” habitable zone for nearly six billion years, more than enough time for life to develop and thrive. Low-mass red giants may actually be very good places to look for life-bearing planets.
But back to the Sun: When all the remaining nuclear fuel is gone, the old star sheds its outer envelope and creates a short-lived but beautiful planetary nebula. What remains of the Sun is a searing-hot but tiny stellar remnant: a white dwarf (WD). Its initial temperature might reach hundreds of thousand Kelvin (decreasing fast at the outset), and its size is just slightly larger than the Earth. It would be surrounded by scorched rocky bodies and somewhat bedraggled distant gas and ice giants with their now volatile-poor moons, and a faraway belt and cloud of comets and the occasional dwarf planets. As it cools to reach the temperature of the cosmic background and becomes a black dwarf in some trillions of years, there is nothing interesting happening around. Or is there?
Many white dwarfs have polluted atmospheres. Instead of just helium, hydrogen, and other elements we could expect there, we see spectral lines of much heavier elements on their surfaces. It means that there is relatively fresh infall of material onto the star, because heavier components tend to sink deeper into the star remnant quite fast.
What can be the source of the material? Asteroids, planetoids? Some orbits could become destabilized, resulting in a crash of the object with the star. In 2015, a disintegrating planetesimal was discovered transiting in front of its host star, WD 1145+017. The star also harbors a dust disk. Insofar, almost twenty elements were discovered in various WD atmospheres, which can give us insights into the composition of their planetary systems. In theory, it could even suggest whether the destroyed planet had plate tectonics, and there is an off-chance of observing traces of long-lost life.
But do these systems need be destabilized indefinitely? Certainly not. There is no reason to presume that WDs in principle couldn’t host planets on stable orbits. And if they do—could some of these worlds harbor life?
Life Under the Tiny White Sun
We’ve already established that white dwarfs are extremely small and hot (at least initially; they cool to a few tens of thousand Kelvin—still a magnitude hotter than the Sun—quickly, and then cool slower). How could such a star host a habitable planet or habitable on the surface, if we for now discount objects harboring oceans under ice?
Astronomer Eric Agol suggested the existence of a WD habitable zone stable over several billion years in his 2011 paper. White dwarfs that have cooled to temperatures of a few thousand Kelvin would radiate “just right” to keep an Earth-like planet on a close-in orbit habitable for three of more billion years, until they’ve cooled too much. Since the planets need to be extremely close to the star to receive enough heat (but not too close to be shattered by tidal forces), their transits in front of the star should be well-observable. What sights could we see from such a planet? It would almost certainly be a tidally locked world, one side bathed in perpetual light and the other drowning in eternal night. From the dayside, the relative size of the star would appear similar to our Sun—it would be much closer, but its absolute size would be like that of the Earth. The light spectrum should allow for photosynthesis and shouldn’t be harmful for organic molecules.
In 2013, Abraham Loeb and Dan Maoz argued that biosignature detection—observing chemical traces associated with life in a planet’s atmosphere—would be easier for WD planets, since due to the negligible size difference, the contrast between the absorption lines in the atmosphere and the background stellar light would be much higher than for planets of main-sequence stars. They simulated a spectrum that could be observed by the James Webb Space Telescope, now slated to be launched in Spring 2019 unless further delays occur. But we need to know more cool WDs in our stellar neighborhood. There is a chance they may be identified in ESA’s star-charting Gaia mission results. The second batch of its data will be released in late April. Afterwards, if new exoplanet hunters such as NASA’s TESS, ESA’s CHEOPS, or ground-based telescopes discover any WD planets, we might soon learn about their atmospheric composition and chances for life.
But there is a slight problem. First, if we presume first-generation and not newly formed planets, they may be in a not-so-habitable state even if volatiles are delivered there by comets and asteroids. Second, as the white dwarf cools, the habitable zone shifts closer to the star—the other way then around main-sequence stars like our Sun. If a planet is initially too close, it may lose its atmosphere and water to the heat and radiation, or become a greenhouse hell like Venus and only then enter the habitable zone without actual habitable conditions. Moreover, tidal heating may greatly increase the likelihood of this pessimistic scenario, or transform the planets into volcanic wastelands. Other planets may start in the habitable zone, perhaps with everything to be conductive for life, but then become frozen balls, unless geochemical processes or life—if present—produce increasingly more greenhouse gases. Finally, could induction heating triggered by WDs’ often powerful magnetic fields prevent close-in planets from being habitable?
However, the whole picture is more complex. Retention of volatiles such as water or atmospheric components depends on many factors such as the planet’s mass and magnetic field, and the path to becoming like Venus is more complicated than just too much sunlight, tides, and greenhouse gases. Being slightly too close to the star may not result in the planet becoming an airless barren rock or an exo-Venus, at least unless its orbital eccentricity is high enough to result in more substantial tidal heating.
As to being slightly too far, it may not be a problem with the right greenhouse gases. The traditional definition of the habitable zone considers carbon dioxide and water vapor, but not other ones such as molecular hydrogen, which makes a very efficient shield against losing heat. A massive planet might also have more inner sources of heat than Earth—more radiogenic heating if it’s composed of more radioactive elements. But it’s a question whether a thick-enough hydrogen atmosphere would survive the red giant phase (probably not, unless the planet was really far away, in which case we can ignore that it orbits the white dwarf at all).
So far, we don’t know the answers, but thanks to currently running and planned surveys, we might soon. After all, it’s our future too. But now it’s time to move to planets of even more extraordinary remnants of stars.
From Supernovae to Pulsars and Black Holes
Indeed, there are more extreme worlds in the universe. In fact, they were the first ever confirmed exoplanets. In 1991, astronomers Alex Wolszczan and Dale Frail detected a peculiar pattern in the radio pulses coming from a relatively nearby pulsar B1257+12 (note: for a pulsar, nearby can mean over 2000 lightyears far away). Pulsars are a class of neutron stars: very compact remnants of stars that used to be many times more massive than our Sun. They measure just around ten miles across and still weigh more than the Sun. Pulsars are rapidly rotating neutron stars emitting powerful radio pulses. These are very regular and make pulsars accurate cosmic clocks. So when some become inaccurate, scientists search for the source of these glitches.
They can be caused by another object gravitationally tugging at the pulsar, making it wobble around their common center of mass. Although the wobble is tiny, we can detect it through timing of the pulses. This is the way that the first exoplanets were found. The original batch of data indicated the presence of two planets around the pulsar B1257+12, one orbiting 0.19 astronomical units (Sun-Earth distances) from the pulsar and approximately twice the mass of our Moon, the other about 0.36 au away and almost four times as massive as the Earth. Later data also revealed a third planet 0.46 au away and slightly more massive than the second one.
Other searches found more planets around pulsars, each completely different in terms of mass and orbit. How did they form? Did they survive the supernova explosion that gave birth to the pulsar, form from the fallback material from the explosion, or from an accretion disk powered by the pulsar devouring a companion star? Could they be captured rogue planets? Or did they form from a disk of interstellar medium enshrouding the tiny neutron star? Since each of the known system is very different from the others, each may have had a different formation scenario!
Could these worlds, orbiting diminutive stellar remnants that emit not just powerful radio pulses, but also strong magnetic fields, X-rays and an intense wind of charged particles, have any chance of harboring life? Despite the extremity of the environment, some works indicate that if massive terrestrial planets had their own magnetic field, they could be shielded from most of the harmful effects and retain a dense enough atmosphere, thus potentially habitable conditions.
But there are too many open questions about pulsar planets. Each formation scenario would lead to a very different planetary composition and structure, which in turn influences the presence (or absence) and character of the intrinsic magnetic field, atmospheric composition, response to the pulsar’s magnetic field and tidal forces, and so on. So even with the same distance from the star and planetary mass, we could end up with wildly varying worlds.
Unfortunately, they will likely remain elusive for a while. They are too few and too far away to warrant observations aimed at their direct detection, and they would have to produce very—some would say unrealistically—strong signatures, such as thermal emissions or ultraviolet/optical auroral effects, in order for us to observe and characterize them. So far, they remain the stuff of speculation—ideal grounds for science fiction writers, aren’t they?
And we still haven’t got to an even wilder, so far purely hypothetical possibility—that planets may orbit black holes. In theory, rotating charged black holes may host stable orbits beneath their event horizon. It’s difficult to wrap one’s head around the concept of a planet unseen by the rest of the universe, with skies lit up by eternally circulating photons, with possibly some causality-breaking events on the daily menu. It’s all very hypothetical, and essentially impossible to test. A planet above the event horizon may seem less unimaginable to us. Interstellar, anyone? A recent paper focused on the question of whether such objects may receive enough energy to host life, and in short, yes, they might, but in the specific case of the conditions described in the movie Interstellar, they would become UV-cooked molten balls. Such worlds will likely remain the domain of science fiction for a very long time.
Off to the End of Time
Perhaps it’s surprising that such extreme worlds have been utilized in science fiction so little. We may mention Robert L. Forward’s novel Dragon’s Egg, which features a beautiful vision of life on a neutron star, but not a planet of such a remnant. Baxter’s Flux is similar in this respect. Niven’s The Integral Trees and The Smoke Ring perhaps come closest to the concept. Additionally, a 1995 TV movie White Dwarf is set on, expectedly, a WD planet—but doesn’t seem to use the setting in a meaningful way. However, there may be more of short fiction (such as “One Face,” by Niven again) that deals with these subjects. And if not so much . . . perhaps someone was just inspired. After all, these extraordinary worlds tell stories.
They tell us a more complex story of the universe than we’ve expected just decades or even years ago. They show that planets can survive or form under unexpected conditions, and at least one is in an extremely old system and may be evidence of planetary formation just a billion years after the Big Bang. They might even increase the chances for life in the universe when it’s much older than ours. Stellar remnants will stay long after star formation will have ceased, cooling slowly for trillions of years. What happens then?
They won’t remain these cooling embers forever. Eventually, their destiny will depend on the rate of expansion of the universe, and the proton decay. Does it occur, and if so, on what timescale? The ultimate fate of our universe hinges on the answers. Will all matter eventually crumble down to subatomic particles, which will evaporate into bits of energy in a universe as homogeneous as it gets, expanding into virtual nothingness? If this notion seems cold and cruel to you, fear not; even if this particular scenario is correct, it’s still countless trillions (of trillions, etc.) years ahead, more unimaginable than the comparatively meager timescales under which the planets of dead stars may or may not remain habitable.
Plenty of time to explore the universe that we have and find out, isn’t it?