With four years of collected data at hand, Kepler scientists will remain busy even with their spacecraft hobbled. We now know that we’re not going to get Kepler back to full working order following the degradation of two of its reaction wheels, but as this report noted on August 19, possibilities remain for scientific studies using the two remaining reaction wheels aided by thrusters to control the spacecraft’s attitude. And as we’re finding out, a ‘two-wheel’ Kepler mission may still offer opportunities, one of the more fascinating of which is our subject today.
The proposed target is white dwarf stars, the remnants of stars whose mass is not high enough to produce a neutron star as they evolve past the red giant phase. A typical white dwarf has a mass similar to that of the Sun, but a volume close to that of the Earth. While Sirius B, at 8.6 light years out, is the closest known white dwarf, eight white dwarfs are believed to be present among the one hundred closest star systems to the Sun. And while we don’t normally think of white dwarfs as capable of sustaining life-bearing planets, maybe we should take another look. A new paper points out that stars like these can provide an energy source for billions of years.
Image: A white dwarf as compared with the Earth. Credit: Ohio State University/Richard Pogge.
To orbit in a white dwarf’s habitable zone requires an orbit in the range of 0.01 AU for temperatures that could support liquid water on the surface to exist. This is a habitable zone that evolves with time, starting off too hot for liquid water and eventually becoming too cold to sustain it, but surprisingly, a white dwarf planet in this kind of orbit could have a maximum of eight billion years of habitability to support whatever life might form there. Lead author Mukremin Kilic (University of Oklahoma) and team calculate an overall habitable zone extending from 0.005 AU to 0.02 AU.
Could such planets exist? Clearly, an expanding red giant will consume its inner planets before contracting into a white dwarf, so planets within 1 AU or less will presumably have to arrive after the red giant phase. But possibilities exist: We’ve found planets orbiting close to the exposed core of a red giant (KOI 55.01 and KOI 55.02) and we’ve even found planets around pulsars. There are models that produce short period planets in billion-year timescales that seem to be applicable to white dwarfs, with planet formation from nearby gas being one scenario and the capture or migration of planets from much further out in the system being another.
Add delivery of water through cometary impacts and the presence of a habitable world in either scenario seems a bit less unlikely. We can also throw into the mix the fact that 30 percent of the white dwarfs near the Sun show metal-polluted atmospheres perhaps caused by the accretion of rocky debris. Indeed, some 4.3 percent have known debris disks, the latter a demonstration that interactions within the system can send asteroids, moons or small planets close to the white dwarf. But if short-period planets around these stars do exist, we have yet to find them, a fact the paper attributes to our lack of observational data for a sufficient number of stars.
Kilic and team argue that Kepler in its two-wheel mode offers an opportunity to run the kind of survey that would find the first exoplanets in a white dwarf habitable zone. From the paper:
If the history of exoplanet science has taught us anything, it is that planets are ubiquitous and they exist in the most unusual places, including very close to their host stars and even around pulsars (Wolszczan & Frail 1992). Currently there are no known planets around WDs, but we have never looked at a suﬃcient number of WDs at high cadence to ﬁnd them through transit observations. It is essentially impossible to ﬁnd Earth-Jupiter size planets around WDs by any other method (Gould & Kilic 2008). If habitable planets exist around WDs, the proposed Kepler imaging survey will ﬁnd them.
The proposed survey would require 200 total days of observing time examining 10000 white dwarfs in the Sloan Digital Sky Survey imaging area, the great benefit being that Kepler’s wide field of view would allow a large number of white dwarfs to be observed at the same time. The researchers believe up to 100 planets will be identified in the habitable zone, an extension of the Kepler planet-hunting charter extended to a new set of targets. The paper continues:
Biomarkers, including O2, on such planets can be detected with the JWST [James Webb Space Telescope]. Hence, even though this is a completely unexplored search area for transiting planets, the scientiﬁc yield of the proposed survey will be enormous.
Exactly so. Remember this about white dwarfs as transit targets. The stars are about the same size as the Earth, so Earth-sized and smaller planets should be easy to detect as they pass in front of the primary. And once a planet in the habitable zone has been identified, the high contrast ratio between the planet and the host white dwarf means that future telescopes should be able to run the biomarker searches mentioned above. Is it possible that the first evidence of life on an exoplanet may come not from a G- or even an M-class system, but a white dwarf?
The paper is Kilic et al., “Habitable Planets Around White Dwarfs: an Alternate Mission for the Kepler Spacecraft,” a Kepler white paper available as a preprint. For more on white dwarf planets, see Habitable Worlds around White Dwarf Stars. Thanks to Antonio Tavani for the pointer to this work.