How does stars’ radioactivity affect the habitability of nearby planets?

Stars are dynamic entities. They emit radiation and charged particles that affect the planets that orbit them. This “space weather” is thus an important factor we should be studying when looking for habitable worlds that are located in other star systems. We spoke with Anna Hughes, an astrophysics PhD candidate at the University of British Columbia. She studies the space weather of the very coldest stars, called ultracool dwarfs, and how this space weather impacts surrounding planets.

What techniques do we have to study the potential habitability of other star systems?

Habitability is a little bit tricky. We don’t have a lot to go off of, so we typically define it by the presence of water. All life on Earth requires water, and liquid water seems to be necessary for life-forming chemical reactions. So, our bare requirement is that a planet needs to be the right temperature to sustain liquid water on its surface. Searches for habitable planets focus on a few things. One is the size of the planet – in order to have liquid water on its surface, it needs to have a surface. So, the planet needs to be small enough to be terrestrial and not a gas or ice giant. Then we need to consider the amount of heat the planet receives from the star. Stars vary in temperature, so the distance range where liquid water can exist – called the habitable zone – varies from star to star. It’s very distant for larger stars, and very close for small stars. A cool thing to note is that a lot of gas giants orbit in the habitable zones of their stars. While gas giants can’t support life themselves, they might have moons that could.

Once we have established that a planet (or moon) orbits in its star’s habitable zone, the rest gets more speculative. The planet will need an atmosphere. We can detect whether it has one by looking for signatures in the planet’s electromagnetic spectrum, or by examining how the light from the star decreases as the planet passes in front of it. These techniques aren’t too sensitive yet. I think so far, we’ve only been able to detect atmospheres of gas giant planets. But future missions like the James Webb Space Telescope will be able to find atmospheres around rocky planets, too. A lot of other factors can impact a planet’s habitability. For example, stars with lots of magnetic activity can strip the atmospheres away from planets and bombard the surfaces with catastrophic radiation – that is where my work comes in.

What are you doing specifically to study the habitability of other star systems?

In my research, I study the radioactivity of the very coldest stars, called ultracool dwarfs. When I say cold, I mean that they’re still thousands of degrees Celsius or Fahrenheit, but they’re much colder than the Sun. Some of them have intense magnetic activity and this is what I’m looking for. I use radio telescopes to search for violent “space weather” – the kind of high-energy radiation and charged particles that cause the auroras on Earth, but thousands of times stronger. Many ultracool dwarfs host planets, so studying how many of them have violent space weather, and getting a sense of how strong that space weather is, is an important part of assessing whether they are generally good hosts for habitable planets.

What advice do you have for people who like to study the habitability of other star systems?

I guess the biggest thing for me is to look at the whole picture. In order to understand what life on a planet might look like, you need to consider all of the factors you can possibly study.