Stars producing more UV light more likely to host exoplanets with life, study finds

Metal-rich stars are more common as the universe gets older, but a new study suggests that they may also make life too difficult on the planets they host.
John Loeffler
A planet orbiting a star
A planet orbiting a star, but can it host life?


When we think of stars that might host life around them, more metal-rich stars might seem better suited, given that they have a much lower UV radiation output than metal-poor stars, but a new study seems to be turning this theory on its head.

The radiation from stars is crucial for the formation of life, ranging from the heat they provide to the energy needed for photosynthesis in plants and other biological processes. But ultraviolet (UV) radiation in too high an intensity can cause major genetic damage, making life (as we understand it, at least) impossible.

This would seem to make metal-rich stars good candidates for hosting exoplanets with life since they are known to have a lower-UV output than metal-poor ones, but a new study indicates that the exact opposite might actually be the case, and that metal-poor stars with temperatures similar to our own might be the better candidates for finding life on exoplanets.

In a new study in Nature Communications, an international team of researchers modeled the atmospheres of Earth-like planets orbiting around stars with near-solar temperatures and a wide range of metallicity to see what effect a star's metallicity had on the amount of ozone (O3) in the resulting atmosphere.

In astronomy, metallicity means the abundance of elements other than hydrogen and helium found in a star, so a higher metallicity means a star has a greater abundance of lithium, silicon, carbon, oxygen, and other elements in its interior. It is also an indicator of a star's age, both in terms of its active life (older stars have had more time to accumulate elements heavier than helium through fusion), as well as its age in the universe. Stars that formed much later in the universe's history will have more elements heavier than helium in the molecular clouds that formed them than stars that formed in the early universe when such elements were far less abundant.

"Paradoxically, whereas stars with higher metallicity, which have appeared later in the history of the Universe, emit less UV radiation, in oxygenated planetary atmospheres the associated stellar radiative spectrum allows less [ozone] formation, which enhances UV penetration, making the conditions on planets orbiting these stars less friendly for the biosphere on land," the study said.

"The relatively low UV emission from the high-metallicity stars can also be a hurdle for the origin of first life on planets with [non-oxygenated] atmospheres," the study adds.

This is because the initial high-UV environment produced by metal-poor stars aids in the development of oxygen-producing organisms that go on to oxygenate the atmosphere. This oxygen then interacts with the star's UV radiation to create ozone, creating a UV barrier to allow more complex life to form in the resulting UV shadow.

"We thus find that the surface of planets orbiting metal-rich stars is exposed to more intense UV radiation than the surface of planets orbiting metal-poor stars. Therefore planets in the habitable zones of stars with low metallicity are the best targets to search for complex life on land," the study concludes.

Key to the search for life on other planets?

One of the reasons it has been so hard to search for life on other planets is that the number of stars in just our galaxy alone is absolutely enormous — estimated to be between 200 and 400 billion individual stars. The more we look, the more we find that many of those stars have exoplanets, and not just one or two, but several.

There could be trillions of exoplanets in our galaxy alone to investigate and so finding markers in host stars that point to where life is most likely to form on the planets that orbit it is essential to filtering that trillion-plus candidate pool into one that is much more manageable to astronomers.

Something like star metallicity is an easy enough marker to identify from a pretty quick glance at a star — how much UV light is it producing — so if this study is confirmed, then stars with low metallicity are the kinds of stars we should be investigating more closely if we want to look for life on other worlds.

And, considering that this is a counter-intuitive finding (stars with high metallicity are producing less UV light), this is an important finding since many astronomers would naturally gravitate towards stars with high metallicity as the most hospitable, possibly setting up many researchers for failure from the start.

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