This graphic shows a 3D map of stars near the sun. These stars are close enough to be prime targets for direct image searches for planets using future telescopes. Blue halos represent stars that have been observed with 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 is equivalent to approximately 3.2 light years). Credit: Cal Poly Pomona/B. binder; Illustration: NASA/CXC/M.Weiss
× near
This graphic shows a 3D map of stars near the sun. These stars are close enough to be prime targets for direct image searches for planets using future telescopes. Blue halos represent stars that have been observed with 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 is equivalent to approximately 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) 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 with the next generation of telescopes aiming to take the first images of planets like Earth.
A team of researchers has studied stars that are close enough to Earth for future telescopes to image 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.
All images of planets will be single points 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 composition of the planets’ surfaces and 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 eliminate 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 Cal 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 obtain images of planets in their habitable zones. These future telescopes include the Observatory for Habitable Worlds 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 investigated which stars might host planets with hospitable conditions for life to form and thrive.
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 radiation 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 can play a key role in maintaining a rich atmosphere like Earth’s.”
The researchers used data available in the archives from almost 10 days of Chandra observations and about 26 days of XMM observations to study the X-ray behavior of 57 nearby stars, some of them with known planets. Most of them are giant planets like Jupiter, Saturn or Neptune, while only a handful of planets or planet candidates can be less than twice the mass of Earth.
There are likely many more planets orbiting stars in the sample, especially ones of Earth-like size, that have so far remained undiscovered. Transit surveys that 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 needed to spot them. This means that the chances of finding transiting planets in a small sample of stars are low; only one exoplanet in the sample was sampled by transits.
The other main technique for detecting planets is by detecting the wobble of a star caused 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 detected in images with the next generation of telescopes, but we do know that observing the weather on them will be valuable and extremely difficult to obtain,” said co-author Edward Schwieterman of the University of California, Riverside. “This X-ray data helps refine and prioritize the target list and could allow the first image of an Earth-like planet to be obtained more quickly.”
