Six highly irradiated terrestrial planets and a star with an X-ray cycle. The study: “Long-term X-ray variability of the multiple-planet host L 98-59: Hints of an activity cycle” by I. Pillitteri (INAF-OAPA) appeared on A&A
A star hosting six terrestrial-type planets and showing intense X-ray emission, with a cyclic behavior on a timescale of about two years, which may have a significant impact on the properties of the planets and their atmospheres.
The magnetic activity of a star manifests itself through a variety of high-energy phenomena that produce X-ray and UV radiation. Studying this activity is important for several reasons, ranging from understanding the processes responsible for the generation of stellar magnetic fields to measuring the level of stellar X-ray emission and its potential impact on the circumstellar environment.
This latter aspect is particularly crucial in the case of exoplanetary systems, especially those orbiting low-mass stars (spectral type M). X-ray radiation emitted by the star and incident on the planets is absorbed by atmospheric gases, triggering a series of processes capable of altering atmospheric chemistry and, in the most extreme cases, heating the atmospheres to the point of causing significant atmospheric evaporation. Around M-type stars, the habitable zone (i.e., the region where a rocky planet may potentially host liquid water on its surface) lies very close to the star, at distances of only a fraction of an astronomical unit (1 astronomical unit corresponds to the average distance between the Earth and the Sun, about 150 million kilometers). At such close distances, the effects of X-ray radiation on planetary atmospheres can be even more dramatic.
Decades of observations of the Sun magnetic activity show that the intensity of its X-ray emission is cyclic, with a periodicity of about 11 years. Is this behavior common among other stars as well? While chromospheric activity can be monitored from the ground through spectroscopic observations—allowing activity cycles to be measured in a large number of stars—X-ray observations must be carried out from space, and long-term observational campaigns targeting individual stars are particularly challenging. For this reason, to date only seven stars are known to exhibit a well-characterized X-ray activity cycle.
The study “Long-term X-ray variability of the multiple-planet host L 98-59: Hints of an activity cycle”, led by astrophysicist I. Pillitteri (INAF – Palermo Astronomical Observatory) and recently published in Astronomy & Astrophysics, presents evidence for a probable two-year X-ray activity cycle in the M-type star L 98-59. This star is known to host a system of six terrestrial-type planets, two of which lie within the habitable zone.
Over the course of the cycle, the X-ray luminosity of L 98-59 varies from about 0.6 to about 10 × 10²⁶ erg/s. Even during the minimum-emission phase, the planets in the system are exposed to an X-ray flux hundreds to thousands of times stronger than that currently reaching the Earth. Such a flux likely has a significant impact on the properties of the planets and their atmospheres and represents a crucial factor to consider when assessing their potential habitability around stars of this type.
The cover image (click here to view it in full) shows, in the upper panels, two X-ray observations of L 98-59 obtained in 2024 and 2025, highlighting the variability of the source. The crosses mark the source positions listed in the public astronomical catalog SIMBAD (L 98-59 is characterized by a rapid proper motion). The lower panel shows the measured X-ray luminosity of the star, overlaid with the periodic variation inferred from the observations.
Mario Giuseppe Guarcello
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