Cold stars with powerful winds complicate habitability

Scientists at the Leibniz Institute for Astrophysics in Potsdam (AIP) have conducted the first systematic characterization of the properties of stellar winds in a sample of cool stars.

Using state-of-the-art numerical simulations, they found that stars with stronger magnetic fields produce more powerful winds. These winds create unfavorable conditions for the survival of planetary atmospheres, thus affecting the possible habitability of these systems.

The sun is among the most abundant stars in the universe known as "cold stars." These stars fall into four categories (type F, G, K, and M) that differ in size, temperature, and brightness. The sun is a fairly average star and belongs to category G. Stars that are brighter and larger than the sun are in category F, while K stars are slightly smaller and cooler than the sun. The smallest and faintest stars are M stars, also known as "red dwarfs" because of the color in which they emit most of their light.

Satellite observations have revealed that, in addition to light, the sun emits a persistent stream of particles known as the solar wind. These winds travel through interplanetary space and interact with the planets of the solar system, including Earth. The beautiful display of auroras near the north and south poles is, in fact, produced by this interaction. However, these winds could also be damaging, as they can erode a stable planetary atmosphere, as was the case on Mars.

While much is known about the solar wind, thanks in part to missions like Solar Orbiter, the same is not true of other cool stars. The problem is that we cannot see these stellar winds directly, which limits us to studying their influence on the thin gas that fills the cavity between the stars in the galaxy. However, this approach has several limitations and is only applicable to a few stars. This motivates the use of computer simulations and models to predict the various properties of stellar winds without the need for astronomers to observe them.

In this context, the first systematic study of the expected stellar wind properties for stars F, G, K and M has been carried out.

To do this, the researchers used numerical simulations employing one of the most sophisticated models currently available, driven by the observed large-scale magnetic field distribution of 21 well-observed stars. The simulations were carried out in the supercomputing facilities of the AIP and the Leibniz Rechenzentrum (LRZ). The findings are published in the Monthly Notices of the Royal Astronomical Society.

The team examined how properties of stars, such as gravity, magnetic field strength, and rotation period, affect the characteristics of the wind in terms of speed or density. Results include a complete characterization of stellar wind properties across spectral types which, among other results, indicate the need to revise previous assumptions about stellar wind velocities when estimating associated mass loss rates from the observations. In addition, the simulations make it possible to predict the expected size of Alfvén’s surface, the boundary between the star’s corona and its stellar wind.

This information is critical to determining whether or not a planetary system may be subject to strong star-planet magnetic interactions, which can occur when the planetary orbit enters or is completely embedded in the Alfvén surface of its host star.

Their findings show that stars with magnetic fields larger than the sun's have faster winds. In some cases, stellar wind speeds can be up to five times faster than the average speed of the solar wind, which is typically 450 km/s. The research obtained an assessment of how strong the winds from these stars are in the so-called "habitable zones," defined as the orbital distances at which rocky exoplanets could sustain liquid water on the surface, provided atmospheric pressure is similar to that of the Earth.

They found milder conditions around F- and G-type stars, comparable to those experienced by Earth around the G-type sun, and increasingly harsh wind environments for K- and M-type stars. Such strong stellar winds affect strongly any potential atmosphere the planet may have.

This phenomenon is well documented in solar physics between rocky planets and the sun, but not in the case of exoplanetary systems. This requires estimates of the stellar wind to assess processes similar to those we see between solar winds and planetary atmospheres. Information on stellar wind was previously unknown for main sequence stars F to M, making this study important in the context of habitability.