Coming up hot: NASA’s Chandra checks for habitability of exoplanets

Using NASA’s Chandra X-ray Observatory and ESA’s (European Space Agency) XMM-Newton, astronomers are exploring whether nearby stars might have habitable exoplanets, based on whether they emit radiation that could destroy the conditions possible for life as we know it. This type of research will help guide observations with the next generation of telescopes aimed at taking the first images of Earth-like planets.

A team of researchers has examined stars that are close enough to Earth that future telescopes could take images of planets in their so-called habitable zones, defined as orbits where planets may have liquid water on their surfaces. Their results were presented at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.

Each image of the planets will be a single point of light and will not directly show surface features such as clouds, continents and oceans. However, their spectra – the amount of light at different wavelengths – will reveal information about the compositions and atmospheres of the planets’ surfaces.

There are several factors that influence what might make a planet suitable for life as we know it. One of these factors is the amount of harmful X-rays and ultraviolet light it receives from its host star, which can damage or even remove the planet’s atmosphere.

“Without characterizing the X-rays from its host star, we would be missing a key element of whether or not a planet is truly habitable,” said Breanna Binder of California Polytechnic State University in Pomona, who led the study. “We need to see what kind of X-ray doses these planets are getting.”

Binder and her colleagues started with a list of stars that are close enough to Earth for future ground-based and space-based telescopes to image planets within their habitable zones. These future telescopes include the Habitable Worlds Observatory and extremely large ground-based telescopes.

Based on X-ray observations of some of these stars using data from Chandra and XMM-Newton, Binder’s team examined which stars might host planets with conditions conducive to the formation and development of life.

The team studied how bright the stars are in X-rays, how energetic the X-rays are, and how much and how quickly they change in X-ray output, for example, due to flares. Brighter and more energetic X-rays can cause more damage to the atmospheres of orbiting planets.

“We have identified stars where the X-ray environment of the habitable zone is similar to or even milder than the one in which Earth evolved,” said Sarah Peacock, a study co-author from the University of Maryland, Baltimore County. “Such conditions may play a key role in maintaining a rich atmosphere like the one found on Earth.”

The researchers used archival data from almost 10 days of Chandra observations and about 26 days of XMM observations to examine the X-ray behavior of 57 nearby stars, some of them with known planets. Most of these are giant planets such as Jupiter, Saturn or Neptune, while only a small fraction of planets or candidate planets may be less than twice the mass of Earth.

There are likely many more planets orbiting stars in the sample, especially those similar in size to Earth, that so far remain undiscovered. Transit studies, which look for small dips in light when planets pass in front of their stars from our point of view, miss many planets because special geometry is required to spot them. This means that the chances of detecting transiting planets in a small sample of stars are low; only one exoplanet in the sample was captured by the transit.

The other main technique for detecting planets is by detecting the wobble of a star caused by orbiting planets, and this technique is primarily sensitive to finding giant planets relatively close to their host stars.

“We don’t know how many Earth-like planets will be discovered in images with the next generation of telescopes, but we do know that observing time on them will be precious and extremely difficult to obtain,” said co-author Edward Schwieterman of the University of California, Riverside. “This X-ray data is helping to refine and prioritize the target list and may allow the first image of an Earth-like planet to be taken sooner.”