Which Stars are Lethal to their Planets?

Many years ago, there was a viral YouTube video called “History of the entire world, i guess,” which has been an endless source of internet memes since its release. One of the most prominent is also scientifically accurate—when describing why animals couldn’t start living on land, the video’s creator, Bill Wurtz, intones, “The Sun is a deadly laser.” 

Early in planetary development, the X-ray and ultraviolet radiation level of a planet’s host stars could sterilize the entire planet’s surface, even if it is in the so-called “habitable zone.” To narrow down the search for potentially habitable planets, the team at the Chandra X-ray Observatory and XMM-Newton telescopes took a look at stars that had planets in their habitable zone and analyzed them for whether the star’s radiation itself might preclude life as we know it from developing there.

Over ten observational days on Chandra and 26 on XMM-Newton, scientists observed 57 stars close enough to Earth to have their exoplanets explored by the next generation of exoplanet-hunting telescopes, such as the Habitable Worlds Observatory. While not all of them had known exoplanets, at least some did. 

However, those exoplanets were typically much larger than Earth, even if they were in the habitable zone. It is much easier to detect giant planets orbiting close to their stars using modern date exoplanet detection techniques like transiting and astrometry. A press release from Chandra notes how many more rocky exoplanets the size of Earth are likely hiding around these stars, but our limited detection methods are not yet capable of finding them.

That isn’t to say we can’t learn much about their host stars, though, and that is where the data from the paper presented to the 244 meeting of the American Astronomical Society in Madison, Wisconsin, comes in. Watching the X-ray emissions of these local stars allowed the team to narrow down what stars to look at for potentially habitable exoplanets, thereby allowing the future powerful planet hunters to focus their observational time on candidates that are more likely to produce results.

Some of the stars in the study were indeed promising, with X-ray exposure similar to, or even less than, that of Earth when life began forming here billions of years ago. The data measured several aspects of the star’s output, including their brightness, how much energy those X-rays pack, and how powerful the star’s flares are. All of those could significantly impact the ability of life to form on any orbiting planets.

Fifty-seven stars is a relatively small sample size. Still, the proof of concept for how Chandra and XMM-Newton can be used to scout potential systems for habitability can be scaled up before any long-term observing mission for the new planet finders—no doubt they will be shortly as HWF and other missions get closer to fruition.

However, Chandra itself is facing budgetary challenges, causing many in the media to speculate that it might soon go “dark.” XMM-Newton itself is almost a quarter century old at this point, and a new joint X-ray mission, XRISM, is facing its own technical challenges, with a stuck door limiting it from observing in some of its potential wavelengths.

With luck, X-ray astronomy will continue to evolve over the next few decades. Part of that mission might be leading the scouting team for one of the most important astronomical searches humanity is currently undergoing.

Learn More:
Chandra Press Room – Coming in Hot: NASA’s Chandra Checks Habitability of Exoplanets
UT – Chandra’s X-ray Vision Combined With JWST Reveals Even More Details About the Universe
UT – A Collection of New Images Reveal X-Rays Across the Universe
UT – Chandra and JWST Join Forces in a Stunning Series of Images

Lead Image:
Illustration of hot exoplanet.
Credit – NASA/CXC/M.Weiss

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