• The amount and kind of radiation from stars can be dangerous for that star’s planets, potentially making them uninhabitable.
• Astronomers measured radiation from 57 nearby stars.
• They found radiation levels much like Earth experiences – or even lower – from some stars. Any planets orbiting these stars, if such planets exist, might be habitable.

Looking for nearby habitable exoplanets

Do any nearby stars have habitable planets? That is, are any nearby worlds inhabited, or capable of being inhabited? By earthly standards, a growing number of known exoplanets could potentially host life. But do they? One big issue is how much radiation they receive from their stars. Too much radiation – or the wrong kind of radiation – could be dangerous for any existing life. Or it could prevent life from starting. On June 12, 2024, researchers released the result of their close examination of 57 nearby stars. They found some of those stars have similar – or less – radiation in their habitable zones than we have here on Earth.

The stars are close enough to our sun that future telescopes could take images of planets orbiting in their habitable zones.

The researchers used NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton satellite to conduct their study. They presented their findings at the 244th Meeting of the American Astronomical Society (June 9-13, 2024).

Measuring X-ray radiation around nearby stars

To examine the 57 stars, the research team examined used archived data from almost 10 days of Chandra observations and 26 days of XMM-Newton observations. They wanted to measure the amount of radiation being emitted by those stars. Such X-ray or ultraviolet radiation can be deadly for life on planets orbiting close to the stars. It can even affect planets in the habitable zone. That’s the region where the temperature on a rocky planet could allow liquid water to exist. But other factors, including radiation, can still affect the overall habitability of a planet. Lead author Breanna Binder of California State Polytechnic University in Pomona said:

Without characterizing X-rays from its host star, we would be missing a key element on whether a planet is truly habitable or not. We need to look at what kind of X-ray doses these planets are receiving.

The researchers chose nearby stars close enough for future telescopes to take images of any planets in the habitable zones. This includes the upcoming Habitable Worlds Observatory and ground-based Extremely Large Telescope. Even then, because the closest stars are still incredibly far away, the planets will look like dots. However, the telescopes can measure the spectra of the light coming from the planets. That can reveal the composition of the planets’ surfaces and atmospheres.

Some nearby stars could have habitable exoplanets

X-ray and ultraviolet radiation can strip the atmospheres off planets too close to their stars. Indeed, this happens with many red dwarf stars. But even in the habitable zone, the radiation could still be deadly for life. The researchers wanted to know the radiation conditions of the 57 stars they studied. As it turned out, the radiation levels for some of the stars was low enough that life could theoretically exist on planets in the habitable zone. Co-author Sarah Peacock at the University of Maryland, Baltimore County, said:

We have identified stars where the habitable zone’s X-ray radiation environment is similar to or even milder than the one in which Earth evolved. Such conditions may play a key role in sustaining a rich atmosphere like the one found on Earth.

Quick Look: Coming in Hot: NASA’s Chandra Checks Habitability of Exoplanets. Video via Chandra X-ray Observatory/ NASA/ YouTube.

More planets?

Currently, astronomers do know of some exoplanets among the group of stars they studied. Most are larger gas giant planets like Jupiter and Saturn. Some are more like Neptune. But astronomers think there are likely many more planets to be found. The largest ones are just the easiest to detect. The transit method is used to find many such planets. But it can also miss many of them because the planet needs to transit – cross in front of – its star as seen from Earth. This means the viewing angle must be just right.

Astronomers also often use doppler spectroscopy technique, aka the radial-velocity method, when searching for exoplanets. This is when they measure the “wobble” a star makes due to the gravitational influence of planets. That method, though, is best suited for finding giant planets orbiting close to their stars, known as hot Jupiters.

But there are also many super-Earth and Earth-sized worlds out there, so it is reasonable to think they could be around some of these 57 stars as well. As co-author Edward Schwieterman at the University of California, Riverside, noted:

We don’t know how many planets similar to Earth 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. These X-ray data are helping to refine and prioritize the list of targets and may allow the first image of a planet similar to Earth to be obtained more quickly.

Bottom line: By measuring radiation levels, astronomers using the Chandra X-ray Observatory and XMM-Newton satellite said some nearby stars could have habitable exoplanets.