A white dwarf is a dead star that slowly cools down until it fades into oblivion. Yet it has been predicted that habitable planets can orbit a white dwarf. If we can somehow detect these planets, would we also be able to spot signs of life?

Scientists have created an artificial spectrum showing that the upcoming James Webb Space Telescope (JWST) will be capable of detecting oxygen and water on an Earth-like planet orbiting a white dwarf.

A white dwarf is the end stage of evolution of a low mass star, and it is tiny compared to its former self. The habitable zone around a white dwarf would therefore have once been located deep within the region of space the star once inhabited, requiring planets to migrate inwards to experience temperatures that are just right for surface liquid water. Infrared observations also have revealed disks of dust surrounding some white dwarfs, which could be the birthplace of a new generation of planets.

Searching for signs of life on a white dwarf exoplanet will involve inspecting the spectral fingerprint of the planet's atmosphere. Exoplanet atmospheres can be detected and analyzed during a planetary transit, when a planet passes in front of a star from our point of view, as the background starlight shines through the planet's atmosphere.

Elements in the atmosphere will absorb some of the starlight, meaning that more light than normal will be blocked at the particular wavelengths associated with that element, giving us a spectrum of the planet.

This technique, called transmission spectroscopy, is difficult to utilize because the parent star is incredibly bright and thus washes out most of the planetary signal. However, if the host star is a white dwarf instead of a main sequence star, then the small stellar radius will result in a very prominent transit signal.

The diameter of the average white dwarf is around 17,000 kilometers, which isn't much bigger than the Earth's diameter of 12,800 kilometers. Therefore, although white dwarfs are dim and hard to detect, it should still be possible to see the signal of an Earth-like planet transiting one.

A sign of life
Certain elements in a planet's atmosphere may indicate the presence of life. Such 'biomarkers" include oxygen and methane, gases that are produced by different forms of life on Earth and would quickly degrade if they weren't constantly being generated.

Some of Earth's biomarkers are prominent in the infrared region of the spectrum, making JWST ideal to search for signs of life on other planets. The JWST, due to launch in a few years time, will be looking in the infrared part of the light spectrum, and it will be able to observe atmospheres on planets that are only a few times the mass of the Earth in the habitable zones of M-type stars (red stars that are cooler than our Sun).

However, the total amount of observing time needed for this far exceeds that which is needed to observe planets around white dwarfs, since the planetary signal is much weaker for the brighter M-dwarfs.

Avi Loeb from Harvard University and Dan Maoz from Tel-Aviv University in Israel decided to test what kind of information they might be able to pry from the atmosphere of a planet orbiting a white dwarf by creating a simulated JWST spectrum. Their synthetic spectrum showed that the oxygen (O2) "A-band" should be easily visible , as well as signatures of water (H2O), assuming they exist on the planet.

"Detecting any of these biomarkers in the atmosphere of an Earth-copy planet around a nearby normal star, using JWST, will be extremely challenging, if not impossible," Maoz told Astrobiology Magazine.

"The difficulty lies in the extreme faintness of the signal, which is hidden in the glare of the 'parent' star. The novelty of our idea is that, if the parent star is a white dwarf, that glare is greatly reduced, and one can now realistically contemplate seeing the O2 biomarker. Detecting other biomarkers will require future space telescopes that are even more ambitious than JWST."

A strong signature of oxygen in a planetary spectrum could indicate that life is present, since oxygen needs to be produced in vast quantities in order to counteract how easily it reacts with other substances.

Biological processes are the main cause for high amounts of oxygen: the 21 percent of oxygen in the Earth's atmosphere is produced by photosynthesis in plants and algae. If life on Earth were suddenly to be extinguished, then all the oxygen would be removed in one or two million years as it combines with rocks and dissolves in the ocean.

The search for elusive planets
No planets have yet been detected orbiting a white dwarf, due to the difficulty in observing these faint stars, however there is some evidence to suggest that such planets might exist. White dwarfs should typically have a pristine spectrum of either hydrogen or helium, as any heavier elements will sink deep within the star. However, many white dwarfs show signs of pollution by heavy elements, possibly due to rocky material in circumstellar disks being perturbed inwards by unseen planets.

Some eclipsing binaries which contain a white dwarf have been observed to have unusual variations in the timing of the eclipses, which could indicate that a planet is present. Planets have also been discovered around pulsars, showing that it is possible for planets to orbit compact stellar remnants.

In order to detect Earth-like planets around white dwarfs, a survey will first need to be performed to select the brightest, nearest white dwarfs suitable for JWST observations. Many white dwarfs will need to be monitored in order to guarantee the best chance of detecting a planet. For instance, if a third of all white dwarfs host an Earth-mass planet within their habitable zones, then 500 white dwarfs would need to be monitored to discover just one transiting Earth.

"We expect to find maybe one or two Earth-like planets that transit white dwarfs, and are observable with JWST, *if* such planets at all exist around white dwarfs," said Maoz.

A survey searching for an O2 biomarker would probably benefit from focusing on white dwarfs that are over three billion years old. It took around two billion years for life on Earth to start producing significant amounts of O2, so neglecting young white dwarfs would turn the focus more on planets where life has had time to evolve. While this is obviously biased, scientists feel it makes more sense to invest limited observing time on the most likely candidates.

Although Earth-like planets are the most interesting targets from an astrobiology perspective, it turns out that they are also the optimum targets when one is looking to detect signs of life on white dwarf planets. A planet with a diameter similar to that of the Earth is just the right size for astronomers to be able to detect a good spectrum of the planet's atmosphere.

If a planet is larger than the white dwarf, the probability of the planet's atmosphere transiting the star will be similar to that of an Earth-size planet. However, the greater surface gravity of the bigger planet will mean that the height of the atmosphere would likely be much lower, meaning less starlight passes through it, thus making it harder to detect.

The advantage to using JWST to observe exoplanetary atmospheres lies in the fact that, since it will be a space-based telescope, it will be liberated from the Earth's atmosphere. If an exoplanet atmosphere is very similar to the Earth's atmosphere, then a ground-based telescope will have great difficulty disentangling it from the Earth's atmospheric spectrum.

(However, if the exoplanet atmosphere is vastly different, then it could be detected amid the Earth's own signal, and large ground-based telescopes could then help to provide measurements.) As we move into the era of searching for biomarkers on extrasolar planets, Earth-like planets around white dwarfs may be the first alien worlds where we can detect such indications of life.

A preprint of the paper is available here. The paper has been accepted for publication in the journal MNRAS.