Before I get into today’s story, which is an interesting study on planets around white dwarfs that Andrew Tribick passed along, I want to say a few words about Japan. Centauri Dreams has many, many readers in that country, and the terrible images and stories coming out of there have haunted me these past few days. The suffering of those displaced by the earthquake and tsunami, and the continued problems in resolving the worsening situation at Fukushima, make it hard to focus on any other topic. Speaking for myself here at Centauri Dreams — and I know I speak for the entire Tau Zero Foundation as well — you Japanese readers remain in our thoughts and prayers, and will continue to do so until these great national wounds are healed.
On the space front, today is the day when MESSENGER enters Mercury orbit. Below is the schedule for the events, which we’ll follow closely as orbital insertion occurs.
White Dwarfs and Potential Planets
But for now let’s talk about white dwarfs, those interesting survivors of Sun-like stars that have gone through the red giant phase and presumably swallowed up planets within roughly Earth’s distance from the Sun. An interesting paper from Eric Agol (University of Washington) takes a look at exoplanet possibilities around white dwarfs, and draws some surprising conclusions. We have, of course, searched for habitable planets primarily around stars that are much younger, assuming that a planetary system that had undergone the transformation of a red giant into a white dwarf would be unlikely to provide a suitable home for life. But Agol isn’t so sure.
Remember the process: Stars like the Sun eventually exhaust their nuclear fuel and at some point lose their outer envelope, leaving only the hot core behind. The core, now a hot white dwarf at temperatures exceeding 100,000 Kelvin, will begin a long process of cooling. A typical white dwarf might be half as massive as the Sun, but not much larger than the Earth in size, and as this NASA article points out, that means it’s extremely dense, perhaps 200,000 times as dense as the Earth itself. When it comes to matter, only neutron stars surpass that density.
Agol points out that the most common white dwarfs have surface temperatures in the range of 5000 K, which leads to his calculation that a planet would need to orbit no closer than about 0.01 AU to be at a temperature where liquid water could exist on the surface. What’s intriguing from the standpoint of finding such planets is that a potentially habitable world like this, Earth-sized or even smaller, would in principle be detectable because of the small size of the host star. The white dwarf, in fact, could be completely eclipsed by a habitable planet that orbits it.
But how does a planet survive the preceding red giant phase? One possibility is that new planets could form out of gases near the white dwarf, especially in binary systems where gravitational interactions could play a helpful role. We know of two neutron stars that have planets that conceivably formed from the disk created after a supernova event. Moreover, the pulsar 4U 0142+61 has been shown to have a circumstellar disk thought to have been formed from supernova debris. Planetary capture or migration can’t be ruled out, either.
Defining a Habitable Zone
I’m going to post Agol’s chart on white dwarf habitable zones (WDHZ) below to illuminate what he has to say. Here the habitable zone is plotted against time as a blue-shaded region, and because the white dwarf is cooling, the region shrinks with time. The planet starts off too hot for liquid water, passes through the white dwarf habitable zone, and then becomes too cold for life.
Image: The WDHZ for MWD = 0.6M⊙ vs. white dwarf age and planet orbital distance. Blue region denotes the WDHZ. Dashed line is Roche limit for Earth-density planets. Planets to right of dotted line are in the WDHZ for less than 3 Gyr. Planet orbital period is indicated on the top axis; white dwarf effective temperature on the right axis. Luminosity of the white dwarf at different ages are indicated on right. Credit: Eric Agol.
Using the WDHZ limits, Agol defines a ‘continuously habitable zone’ (CHZ) as a range of orbital distances habitable for a minimum duration. Choosing a minimum duration of 3 billion years produces a continuous habitable zone within 0.02 AU, so we have a three billion year period for the development of life at that distance. The author comments on the consequences:
…the range of white dwarf temperatures in the portion of the CHZ within the WDHZ is that of cool white dwarfs, ≈ 3000–9000 K (right hand axis in Fig. 1), similar to the Sun. At the hotter end higher ultraviolet flux might affect the retention of an atmosphere, these planets would need to form a secondary atmosphere, as occurred on Earth. Excluding higher temperature white dwarfs only slightly modifies the CHZ since they spend little time at high temperature. Cool white dwarfs are photometrically stable…, which is critical for finding planets around them.
Finding a White Dwarf Planet
An interesting prospect indeed, one that Agol further explores by simulating sky surveys that could find such planets. Among the latter calculations, it’s interesting to note that the GAIA mission will observe 200,000 disk white dwarfs between 50 and 100 times each, making the detection of a white dwarf with a habitable planet a real possibility. Even more likely are the prospects for the Large Synoptic Survey Telescope, a planned wide-field survey in Chile.
And what would life be like on a planet orbiting in the habitable zone of a white dwarf?
The most common white dwarf has Teff [effective temperature] ≈ 5000 K, close to that of the Sun; consequently, inhabitants of a planet in the CHZ will see their star as a similar angular size and color as we see our Sun. The orbital and spin period of planets in the CHZ are similar to a day, causing Coriolis and thermal forces similar to Earth. The night sides of these planets will be warmed by advection of heat from their day sides if a cold-trap is avoided… Transit probabilities of habitable planets are similar for cool white dwarfs and Sun-like stars, but the white dwarf planets can be found using ground-based telescopes… at a much less expensive price than space-based planet-survey telescopes.
This is a provocative paper, one that jolts us into thinking about habitable zones in places where we hadn’t thought of looking before. Yet as we’re finding in our exoplanet research, the universe keeps yielding surprises, and a habitable planet around a white dwarf may not be so bizarre after all. Does anyone know of any science fiction writers who have tackled such a scenario? If so, do let me know. Agol’s paper is “Transit Surveys for Earths in the Habitable Zones of White Dwarfs,” in press at the Astrophysical Journal Letters and available as a preprint.
Addendum: See the comments below for a link to a discussion of white dwarf planets that I was hitherto unaware of.