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NASA’s Chandra X-ray space telescope has created a three-dimensional map of stars near the sun that could help astronomers search for exoplanets that could harbor life.
The map created by Chandra, which is celebrating its 25th year in orbit but faces a tight budget, could help scientists figure out which exoplanets their telescopes should be pointed at in the future to search for habitable conditions.
The stars mapped by the telescope are arranged in concentric rings around the sun, at distances between 16.3 light-years and 49 light-years. This is close enough that the telescopes can pick up wavelengths, or “spectra,” of light from planets in the habitable zones of these stars. The habitable zone, or “Goldilocks zone,” is a region around a star that is neither too hot nor too cold to allow liquid water to exist on the surface of a world.
Spectra from these planets, created by starlight passing through their air, can reveal surface features such as continents and oceans, and atmospheric features such as clouds and chemical composition.
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Chandra’s X-ray capability is key in choosing which planets to probe for possible habitability. High-energy light, such as X-rays and ultraviolet radiation, can strip a planet of its atmosphere and also disrupt its habitability by breaking down complex molecules needed as the building blocks of living things.
So if Chandra sees a planet under intense X-ray bombardment, scientists might conclude that it’s not the best world to study in the search for alien life.
“Without characterizing the X-rays coming from its host star, we would be missing a key element of whether a planet is truly habitable,” Breanna Binder of California State Polytechnic University, who led the team behind the new map, said in a statement. “We need to look at what kind of X-ray doses these planets are receiving.”
X-rays are bad news for life, even in Goldilocks zones
Binder and his colleagues first created their map with a list of 57 stars that are close enough to our Solar System that future telescopes in space, such as the Habitable Earths Observatory, and ground-based telescopes, such as the Extremely Large Telescope (ELT), will be able to image planets orbiting in their Goldilocks zones.
However, just because a planet is in the habitable zone doesn’t guarantee it’s habitable. Venus and Mars lie on either side of Earth in the Sun’s habitable zone, but the Martian surface doesn’t appear to be hospitable to life as we know it, and superheated Venus is completely hostile to it.
So to narrow down their list, the team used data from 10 days of Chandra observations and 26 days of observations from the European Space Agency’s (ESA) XMM-Newton space telescope to see how bright stars were in X-rays. They then determined how energetic those X-rays were and how quickly the stars’ X-ray emission was changing.
The scientists concluded that the brighter and more energetic the X-rays, the more likely it is that any orbiting exoplanet has severely damaged or lost its atmosphere entirely.
“We have identified stars where the X-ray radiation environment of the habitable zone is similar to or milder than the environment in which Earth evolved,” explained team member Sarah Peacock of the University of Maryland. “These conditions may play an important role in sustaining a rich atmosphere similar to that found on Earth.”
Some of the stars the team studied are known to be orbited by exoplanets with masses and sizes similar to the solar system giants Jupiter, Saturn, Neptune and Uranus, while a handful of candidates have masses around half that of Earth.
There may be planets in these systems that are more suitable for Earth’s mass and size that have not yet been discovered.
Earth-sized planets in these systems may be missed by the most reliable exoplanet detection method, the transit method, which relies on a planet passing, or “crossing,” the face of its star, causing a small decrease in the output of starlight in the process.
This depends on a planet coming between its star and Earth, meaning some systems are not oriented correctly to see transiting worlds. The technique is better at spotting massive planets close to their stars, so smaller worlds orbiting relatively far away may be missed.
The other primary exoplanet detection technique, the radial velocity method, relies on detecting the “wobble” caused by a planet as it orbits its star and gravitationally tugs on it. Again, this method favors large planets that are close to their stars, which create a more significant wobble.
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“We don’t know how many Earth-like planets will be imaged with next-generation telescopes, but we do know that observations of them will be valuable and extremely difficult to obtain,” concluded team member and University of California, Riverside researcher Edward Schwieterman. “These X-ray data help refine and prioritize the target list and could lead to the first image of an Earth-like planet more quickly.”
The team’s research was presented at the 244th American Astronomical Society meeting in Madison, Wisconsin.