The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b

Abstract

In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts planetary atmospheres. LHS 1140b is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs. We present the results of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV and NUV flux of a sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution. No significant variation of the NUV flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV flux <2% with respect to that of the present-day Earth, while the FUV/NUV ratio is ~100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the GALEX FUV band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.

arXiv comment6 pages, 4 figures, 2 tables. Accepted for pubblication in Astronomy & Astrophysics

References in corpus (37)

• Gaia Data Release 2. Summary of the contents and survey properties
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Habitable Zones Around Main-Sequence Stars: New Estimates
• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• Habitable planets around the star Gl 581?
• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life
• Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
• The MUSCLES Treasury Survey I: Motivation and Overview
• The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: II. The Field
• The Structure of the Local Interstellar Medium V: Electron Densities
• A temperate rocky super-Earth transiting a nearby cool star
• A Re-appraisal of the Habitability of Planets Around M Dwarf Stars
• The Habitability of Planets Orbiting M-dwarf Stars
• The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
• The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present
• Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets
• The Solar Neighborhood XXXVII: The Mass-Luminosity Relation for Main Sequence M Dwarfs
• Tidal Locking of Habitable Exoplanets
• The UV and X-ray activity of the M dwarfs within 10pc of the Sun
• Abiotic O${}_2$ Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?
• High Stellar FUV/NUV Ratio and Oxygen Contents in the Atmospheres of Potentially Habitable Planets
• The Origin of RNA Precursors on Exoplanets
• The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs
• Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
• The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-Up
• UV habitable zones around M stars
• A path towards understanding the rotation-activity relation of M dwarfs with K2 mission, X-ray and UV data
• New rotation period measurements for M dwarfs in the southern hemisphere: an abundance of slowly rotating, fully convective stars
• Time-resolved Ultraviolet Spectroscopy of the M-dwarf GJ 876 Exoplanetary System
• A second terrestrial planet orbiting the nearby M dwarf LHS 1140
• A scenario of planet erosion by coronal radiation
• Living with a Red Dwarf: Rotation and X-ray and Ultraviolet Properties of the Halo Population Kapteyn's Star
• HAZMAT III: The UV Evolution of Mid- to Late-M Stars with GALEX
• Enhanced interplanetary panspermia in the TRAPPIST-1 system
• The Impact of Stellar Distances on Habitable Zone Planets
• HAZMAT II: Ultraviolet Variability of Low-Mass Stars in the GALEX Archive
• Minimizing the bias in exoplanet detection - application to radial velocities of LHS 1140

Cited by in corpus (9)

• Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b
• Habitability and Spectroscopic Observability of Warm M-dwarf Exoplanets Evaluated with a 3D Chemistry-Climate Model
• Simultaneous Optical Transmission Spectroscopy of a Terrestrial, Habitable-Zone Exoplanet with Two Ground-Based Multi-Object Spectrographs
• Detectability of biosignatures on LHS 1140 b
• Hot Rocks Survey III: A deep eclipse for LHS 1140c and a new Gaussian process method to account for correlated noise in individual pixels
• Planetary parameters, XUV environments and mass-loss rates for nearby gaseous planets with X-ray detected host-stars
• Implications of Abiotic Oxygen Buildup for Earth-like Complex Life
• A reanalysis of the LHS 1140 b atmosphere observed with the Hubble Space Telescope
• Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone