Directly imaging planets around other stars is the primary goal of future observatories such as HWO and ELT. But to determine whether a planet has an atmosphere and can support life, we need to know precisely how much X-ray and ultraviolet radiation its star emits. Such radiation heats the upper atmosphere, drives chemical reactions, and can strip gases into space. For most priority targets, these data are lacking: of the 164 stars in the TSS25 list, only 51 have X-ray observations, and just nine have full ultraviolet coverage. This work fills the gap for 12 of the most studied systems at the forefront of observation.
The researchers gathered archival data from the XMM-Newton, Chandra, and Hubble space telescopes. X-ray spectra were modeled with the APEC code, while ultraviolet spectra used the PHOENIX code, which reconstructs the unobservable extreme ultraviolet from emission lines. For each star, they fitted the best-matching model and merged it with real data into a continuous spectrum from X-rays to radio. For Lalande 21185, whose X-ray flux varied by a factor of 27 between two epochs, they built two spectral versions.
Nearly all 12 stars turned out to be more active in X-rays and extreme ultraviolet than the present-day Sun, with fluxes in the habitable zone 10–100 times higher. Only four stars, Delta Pavonis, 55 Cancri A, GL 183, and GJ 832, offer conditions close to solar. Red dwarfs like Proxima Centauri emit relatively more ultraviolet than Sun-like stars, altering atmospheric chemistry. Estimates of oxygen loss over a billion years show that for most Sun-like stars, losses don't exceed tens of bars, but for the most active red dwarfs, they could reach the equivalent of Earth's ocean. Comparisons with independent empirical relations confirmed the reliability of the reconstructed ultraviolet fluxes.
This work shows that high-energy radiation is a crucial filter in selecting targets for the search for life. Unlike previous studies focused on red dwarfs, this one covers stars from F9 to M5.5, including those that will be directly observed. It is the first systematic check of the radiation environment for stars with visually resolvable habitable zones, and it demonstrates that the sample is biased toward more active stars, meaning quieter candidates have yet to be found.
The main next step is to expand the sample. Of the 164 HWO priority targets, only 19 have quality X-ray spectra, and only nine have full ultraviolet coverage. New observations are needed, both single-epoch to establish baseline activity levels and multi-epoch to assess variability. Particularly important are old, slowly rotating Sun-like stars, potentially the calmest havens for life.
The results will directly influence mission planning for HWO and ELT, helping calculate coronagraph exposure times and select stars for detailed exoplanet atmosphere modeling.
Observations of ten stars with Hubble are planned under program GO 18001 to fill gaps in ultraviolet data. Methods must also be developed to account for activity cycles when interpreting non-contemporaneous observations.
For stellar astrophysics, the typical activity level of Sun-like stars remains an open question: the Sun might be anomalously quiet, or it might be typical. This work shows that the studied sample is biased toward active stars, and systematic surveys are needed for an answer. For planetary science, the key unresolved problem is how high-energy radiation drives long-term atmospheric evolution and whether it can completely strip a planet of water.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study compiles panchromatic spectra for 12 priority stars with potentially habitable zones. Using archival X-ray and ultraviolet data from XMM-Newton, Chandra, and Hubble, the authors modeled the radiation from X-rays to radio and compared it with the Sun's. It turns out that most stars irradiate their planets 10–100 times more strongly than the Sun does the Earth today, but this does not necessarily preclude habitability.
Where its strength is
A strong point is the systematic approach to building complete spectral energy distributions for stars of different types, including those that will be targets for direct observations. This provides a quantitative basis for assessing atmospheric loss and chemistry, not just qualitative conclusions.
Where it could go next
Worth trying
Why this may matter
Such spectral passports will become the foundation for planning observations and interpreting data from future telescopes, as well as for understanding which worlds have a chance to retain their atmospheres and support life.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
The star Lalande 21185 changed its X-ray flux by a factor of 27 over three years, as if the Sun suddenly became dozens of times brighter in X-rays for no apparent reason. Such variability is typical of red dwarfs and greatly complicates assessing the habitability of their planets.