Low Mass Stars

Making Exoplanet Searches Easier Since 2004

Finding Alien Worlds

An "exoplanet" is any planet that orbits a star other than the Sun. There are billions of them in our galaxy alone, but they are very difficult to find - even when we restrict the search to our local neighborhood! That's because they don't actually emit their own light. They only reflect the light given off by their host star, which makes them very dim. Most exoplanets are discovered using indirect detection techniques, such as measuring the radial velocity of the host star, or watching the sky for an exoplanet transit.

We can learn much more about the properties of an exoplanet if we can actually take a picture of it. This is called "direct imaging." It's very difficult to do, because the light from the planet (which is really just reflected light from the star) is usually washed out by the light from the star itself. There are three things you can do to combat this problem. First, you can use a coronograph to block out the light from the star. Of course, you risk blocking out some of the planets orbiting close to the star if you do that. Second, you can take pictures of exoplanets in infrared wavelengths, since the stars will be dimmer and the planets will be brighter. Third, you can restrict your search to low-mass stars.

Low mass stars are dim, so the contrast between star and planet is favorable. Plus, low-mass stars make up ~50% of all stars in our galaxy, so they should be easy to find. We also need to find low mass stars that are young, preferably younger than 100 million years. That's because the exoplanets we're looking for are giant planets (1-10 times the mass of Jupiter) that are still in the process of forming. Although planets don't create their own light through nuclear fusion, they can generate heat (and therefore thermal emission) by contracting themselves. This makes them a bit brighter than the typical planet. But these giant planets will get dimmer as they cool off, so the younger they are, the  brighter they will be. This is where my research comes in. I search the skies for young, low-mass stars that would be good targets for direct imaging surveys by planet hunters. Finding low-mass stars is easy, but determine whether they are actually young is not.

How do we find the baby planetary systems?

There are several ways to (approximately) determine the age of a star, but few of them actually work on low-mass stars. Still, there are some options left to us. First, you can look at the motions of a star to see if you can assign it to a stellar association. Stellar associations are groups of stars that were born together, so they are all the same age. It is often easier to figure out the age of a whole group (since you have more data to work with) than it is to figure out the age of an individual star. But if you can figure out if a star belongs to a particular group, you know it has to be the same age as the other group members.

Second, you can look for lithium in the atmosphere of the star. Lithium is found all over the galaxy, but it is burned up easily in the centers of stars. If you see any lithium left in a star, that means that the star must still be fairly young. 

I try to accomplish both of these goals by taking optical spectra of nearby, low-mass stars. What does that mean? Optical means that I am using telescopes that can see these stars in visible light (rather than ultraviolet or infrared light). Taking spectra means that I am splitting up the starlight into its composite colors and measuring how bright the light is at different wavelengths. When I do this, I can also detect the telltale signatures of different elements, which leave fingerprint-like imprints on the stellar spectrum.

Identifying Low-Mass Stellar Hosts

Finally, how do I know that I am even looking at low-mass stars? First, I look at the color. Low-mass stars are cooler, and therefore redder than other stars. But sometimes that's not enough. After all, a star can look red if it's surrounded by dust, even if it's a blue star (e.g. Vega). So I need some other piece of evidence. As it turns out, low-mass stars are very magnetically active. They are constantly erupting in giant stellar flares, much larger than the storms that our own Sun undergoes. Fortunately for astronomers, this turbulent activity produces a ton of ultraviolet radiation. So if we want to find low-mass stars, we need to find stars that look red, but which emit substantial amounts of light at ultraviolet wavelengths.