This graphic shows a three-dimensional map of the stars near the Sun. These stars are close enough that they could be prime targets for direct imaging of planets with future telescopes. The blue halos represent stars that were observed by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. The yellow star in the center of this diagram represents the position of the sun. The concentric rings show distances of 5, 10 and 15 parsecs (one parsec corresponds to about 3.2 light years). Credit: Cal Poly Pomona/B. Binder; Illustration: NASA/CXC/M.Weiss
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This graphic shows a three-dimensional map of the stars near the Sun. These stars are close enough that they could be prime targets for direct imaging of planets with future telescopes. The blue halos represent stars that were observed by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. The yellow star in the center of this diagram represents the position of the sun. The concentric rings show distances of 5, 10 and 15 parsecs (one parsec corresponds to about 3.2 light years). Credit: Cal Poly Pomona/B. Binder; Illustration: NASA/CXC/M.Weiss
Using NASA’s Chandra X-ray Observatory and ESA’s (European Space Agency’s) XMM-Newton, astronomers are investigating whether nearby stars could host habitable exoplanets, based on whether they emit radiation that could potentially destroy the conditions for life as we know it. This type of research will help guide observations using a new generation of telescopes aimed at taking the first images of planets like Earth.
The team of researchers studied stars that are close enough to Earth that future telescopes could take pictures of planets in their so-called habitable zones, defined as orbits where planets could have liquid water on their surfaces. Their results were presented at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.
Any planet images will be single points of light and will not directly depict surface features such as clouds, continents and oceans. However, their spectra – the amount of light at different wavelengths – will reveal information about the composition of the planets’ surfaces and their atmospheres.
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 the X-ray characterization of its host star, we would be missing a key element as to whether or not the planet is truly habitable,” said Breanna Binder of California State Polytechnic University, Pomona, who led the study. “We need to look at what X-ray doses these planets are getting.”
Binder and her colleagues started with a list of stars that are close enough to Earth that future ground-based and space-based telescopes could acquire images of planets in their habitable zones. These future telescopes include the Habitable Worlds Observatory and the ground-based Extremely Large Telescopes.
Based on X-ray observations of some of these stars, using data from Chandra and XMM-Newton, Binder’s team investigated which stars could host planets with hospitable conditions for life to form and thrive.
The team looked at 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 environment in which Earth evolved,” said study co-author Sarah Peacock of the University of Maryland, Baltimore County. “Such conditions may play a key role in maintaining a rich atmosphere like Earth’s.”
The researchers used archived data from nearly 10 days of Chandra observations and about 26 days of XMM observations to examine the X-ray behavior of 57 nearby stars, some of which have known planets. Most of them are giant planets like Jupiter, Saturn or Neptune, while only a handful of planets or planet candidates could be less than about twice as massive as Earth.
There are likely many more planets orbiting stars in the sample, especially those similar in size to Earth, that remain undetected so far. Transit studies that look for tiny dips in light from our perspective as planets pass in front of their stars miss many planets because special geometry is needed to spot them. This means that the chance of detecting transiting planets in a small sample of stars is low; only one exoplanet in the sample was captured by transits.
Another major planet detection technique is the detection of stellar wobbles induced by orbiting planets, and this technique is mainly 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 from the next generation of telescopes, but we do know that time observations on them will be rare and extremely difficult to obtain,” said co-author Edward Schwieterman of the University of California, Riverside. “This X-ray data helps to refine and prioritize the target list and may allow for a faster acquisition of the first image of an Earth-like planet.”