NASA’s Chandra X-ray Space Telescope has created a three-dimensional map of stars close to the sun that could help astronomers search for alien planets capable of supporting life.
The map created by Chandra – the astronomer who just celebrated his 25th anniversary in orbit but is facing worrying budget cuts – could tell scientists which exoplanets they should point future telescopes at to search for habitable conditions.
The stars mapped by the telescope are arranged in concentric rings around the Sun, at distances between 16.3 and 49 light-years. That’s close enough that telescopes could collect wavelengths of light, or “spectra,” from planets in the habitable zones of those stars. The habitable zone, or “Goldilocks zone,” is a region around a star that is neither too hot nor too cold for liquid water to exist on the surface of a world.
The spectra of these planets, created when starlight shines through their air, could potentially reveal surface features such as continents and oceans, as well as atmospheric properties such as clouds and chemical composition.
Related: Happy 25th anniversary, Chandra! NASA celebrates with 25 stunning images from its flagship X-ray observatory
Chandra’s X-ray capability is crucial in selecting planets to study for possible habitability. High-energy radiation such as X-rays and ultraviolet rays can destroy a planet’s atmosphere and also break down the complex molecules needed as the building blocks of living things, thus destroying the planet’s habitability.
So if Chandra discovers a planet that is under strong X-ray bombardment, scientists can conclude that it is not the best planet to search for extraterrestrial life.
“Without characterizing the X-rays from its parent star, we would be missing a key element that tells us whether a planet is truly habitable or not,” said Breanna Binder of California State Polytechnic University, the leader of the team behind the new map, in a statement. “We need to study what kind of X-rays these planets receive.”
X-rays are bad news for life, even in Goldilocks zones
Binder and his colleagues created their map by starting with a list of 57 stars that are close enough to our solar system that future telescopes in space, such as the Habitable Worlds Observatory, and on land, such as the Extremely Large Telescope (ELT), could image planets orbiting in their Goldilocks zones.
However, just because a planet is in the habitable zone does not guarantee that it is habitable. Venus and Mars are both in the habitable zone of the Sun, on either side of the Earth, but the Martian surface appears unsuitable for life as we know it, and overheated Venus is downright hostile to life.
To narrow down their list, the team used data from 10 Chandra observations and 26 days of observations from the European Space Agency’s (ESA) XMM-Newton space telescope to see how bright the stars are in X-rays. They then determined how energetic those X-rays are and how quickly the stars’ X-ray emission changes.
The scientists concluded that the brighter and more energetic the X-rays, the greater the likelihood that the orbiting exoplanets have suffered severe damage to their atmospheres or have lost their atmospheres altogether.
“We have identified stars where the X-ray environment of the habitable zone is similar to or even milder than that of Earth,” explained team member Sarah Peacock of the University of Maryland. “Such conditions could play a key role in maintaining a rich atmosphere like Earth’s.”
Some of the stars studied by the team are already known to be orbited by exoplanets with masses and sizes comparable to the solar system giants Jupiter, Saturn, Neptune and Uranus. A handful of candidates have less than half the mass of Earth.
These systems may also contain planets whose mass and size are more compatible with that of Earth, but which have not yet been discovered.
Earth-sized planets in these systems may have been missed by the most reliable method of exoplanet detection, the transit method. This technique requires a planet to cross, or “transit,” the surface of its star, causing a slight dip in the starlight’s radiance.
This depends on whether a planet comes between its star and Earth, meaning some systems simply aren’t aligned correctly to see worlds using the transit method. The technique is better suited to detecting massive planets close to their star, so smaller worlds orbiting relatively far away could be missed.
The other main method for detecting exoplanets, the radial velocity method, relies on detecting the “wobble” a planet makes as it orbits and gravitates toward its star. This method also favors massive planets that are close to their star, which produce a stronger wobble.
“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,” concluded team member and researcher at the University of California, Riverside, Edward Schwieterman. “This X-ray data will help refine and prioritize the list of targets and could lead to the first image of an Earth-like planet being produced more quickly.”
The team’s research was presented at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.
