Dead Stars: Good Exoplanet Targets?

Planets with Earth-like atmospheres might be easier to detect around white dwarfs than normal stars.

In the last decade, astronomers have gone from barely detecting a few hot Jupiter exoplanets to observing small rocky planets just larger than Earth around low-mass stars. The search for Earth's twin is ongoing, but astronomers are already asking the next question: Once Earth's twin is found, how do we know if it is home to life?

A pair of astronomers has a new prediction that suggests our best chance for identifying signs of life on other planets might not be where we usually look. Abraham Loeb (Harvard University) and Dan Maoz (Tel-Aviv University, Israel) recently published a study that suggests white dwarfs, not main-sequence stars, may be our best chance for observing signs of life on planets around other stars.

White dwarfs are the remnants of low-mass stars like our own Sun. They are the small, dense embers of a stellar core, containing about 60% of the original star’s mass in an orb roughly the size of Earth. (It would take more than 100 Earths lined up in a row to traverse the Sun’s diameter.) These stars cool slowly, maintaining stable temperatures close to that of the Sun for more than 3 billion years. Recent studies have revealed rocky material in disks around white dwarfs, as well as polluting the dead stars’ atmospheres, which suggests the white dwarfs swallowed up infalling asteroids or planets. While planets in Earth-like orbits probably wouldn't survive the bloated red giant phase of a white dwarf's predecessor, planets that migrated inward during the star’s death throes might receive enough warming radiation to create life-sustaining conditions on their surfaces.

To detect the presence of life on other planets, astronomers are looking for molecules that can only form in the presence of life. Molecular oxygen is the strongest biosignature, as it is mainly produced by green algae and large plants on land. The amount of oxygen in Earth's atmosphere rose sharply after vegetation appeared, jumping from a few percent to 20-30%. If all oxygen-producing life disappeared from our planet, molecular oxygen would disappear from the atmosphere after a million years or so. Thus, the detection of oxygen in another planet's atmosphere would be a strong indication that life exists on that planet.

The detection of atmospheres on other planets is quite difficult and can only currently be done for transiting exoplanets. As the planet passes between Earth and the star, some of the starlight passes through the exoplanet’s atmosphere, where certain wavelengths are absorbed by the compounds in the planet’s gaseous envelope. These absorption lines are telltale signals of what’s in the planet’s atmosphere. By comparing those spectra to what’s observed when the planet is hidden behind the star (when just the pristine stellar spectrum appears), astronomers can tease out the composition of the exoplanet atmosphere.

Only a handful of exoplanets have been studied in this fashion, since the signal from the planet is usually about 10,000 times weaker than the background starlight. Detecting such a minute drop in starlight is a bit like trying to spot a dead light bulb on the Eiffel Tower at night.

But this detection would be much easier for a planet orbiting a white dwarf, Loeb and Maoz say. White dwarfs’ tiny size and dim light output would make an exoplanet’s atmosphere easily detectable — especially the important signal from molecular oxygen, if the amount of atmospheric oxygen roughly matches Earth’s. They simulated the spectrum of an exoplanet as seen by the upcoming James Webb Space Telescope (JWST), which will study exoplanet atmospheres as one of its primary science goals. Loeb and Maoz found that the observing time needed to tease oxygen’s signature out of the atmosphere of a white-dwarf-orbiting planet is a factor of ten lower than for other planets, making white dwarfs a fertile hunting ground for detecting life-sustaining worlds outside of our solar system. Detecting water vapor (which has already been done around other planets) would be even easier.

There is a caveat to white dwarf worlds. Because white dwarfs cool down over time, the radius of their habitable zones shrinks as they cool. A planet that was once in the Goldilocks zone will be outside it a couple billion years later.

Still, these burnt-out stellar embers could be the best targets for studying exoplanet atmospheres. They may just end up as one of the first targets for JWST.

Reference: A. Loeb and D. Maoz. "Detecting bio-markers in habitable-zone earths transiting white dwarfs." arXiv.org January 21, 2013.