Science Sunday: Exoplanets!
It seems like nearly every week there’s a new announcement that astronomers have found another exoplanet, a planet orbiting a different star. Sometimes that planet is a Hot Jupiter, sometimes it’s a Super-Earth, sometimes it’s a planet made entirely of diamond or burning ice. Exoplanets are weird, incredibly diverse, and always fascinating. But there are plenty of ice-burning questions, such as, how many exoplanets have we observed? How many are out there? How exactly do we find them?
Exoplanets have a long history. People have been claiming to detect exoplanets for almost two hundred years. Many astronomers in the nineteenth and twentieth centuries claimed to be able to detect planets orbiting around nearby star systems. Of course, all of these claims turned out not to be true.
The first confirmed detection of an extrasolar planet occurred in 1992. Two astronomers, Aleksander Wolszczan and Dale Frail, announced that they had discovered two planets orbiting a pulsar. Three years later, the first planet orbiting a main-sequence star (like ours) was observed. Since then, 927 exoplanets have been discovered, orbiting 715 stars. There are still thousands of possible planet detections that have yet to be confirmed, and billions more awaiting discovery.
But how common are they? Does every star have a system of planets, or is a star lucky to have even one? Well, it turns out that, on average, there’s about one planet per star in our galaxy. That means there’s over 100 billion planets in the Milky Way alone. Of course, that’s an average, so there are likely many stars without planets, and some stars with dozens of planets and hundreds of moons (five people will get this joke).
But how many planets can support life? Well, this one is a bit tougher, because there are a number of factors that astronomers have to consider. For instance, the planet has to be a certain size, it has to orbit in the star’s habitable zone (the region where liquid water can exist), and it has to contain all the elements and compounds needed for life. It’s difficult to measure all the variables, so at this point all we have are estimates ranging from 600 million to 100 billion.
So how do we find exoplanets? They’re small and dim, and they are often obscured by their parent star. In most cases, it’s impossible to see the planet directly, and even then, we pretty much already have to know they’re there in order to image them. Fortunately for us, astronomers are really freakin’ clever. We don’t have to see the planet to know it’s there, we just have to measure the planet’s effect on it’s star, which we can see.
For instance, when a planet orbits a star, the star isn’t stationary. The star and the planet both orbit around the center of mass of the system, so the star moves as well, because it’s pulled by the planet’s gravity. If we observe a star moving over time, we can conclude that something is orbiting it, and by measuring how much it moves, we can determine how massive it is. This method is called astrometry.
Astrometry is the oldest method of detecting exoplanets, dating back to the nineteenth century. It’s also one of the least successful, because the planet involved has to be extremely massive to detect. Of course, that’s not going to stop astronomers from trying. In a few months, the European Space Agency is launching GAIA, the Global Astrometric Interferometer for Astrophysics. It’s mission is to accurately map the entire Milky Way Galaxy, and provide precise measurements of all the stars in the night sky. It will also be watching those stars to detect any movement that may be the result of exoplanets. It’s expected to detect tens of thousands of exoplanets over its lifetime.
GAIA isn’t the only space telescope tasked with finding exoplanets. In 2009, NASA launched the Kepler Space Telescope, designed to do one thing: Watch about 150,000 stars, and monitor their brightness. Kepler is waiting for a planet to pass in front of its star, dimming it ever so slightly. This is the transit method of exoplanet detection. Kepler is actually very good at this; it’s so precise that it can detect someone waving a lantern on Mars.
So far, Kepler has been a resounding success. It’s managed to detect 134 exoplanets, as well as identify over three thousand possible candidates. The mission is still ongoing, and there’s a lot of data that has yet to be crunched. In fact, you can help. Much of the raw data has been released to the public, and the organization Zooniverse has created a way for everyone to participate in the science. If you want, you can go here and join the thousands of people looking for exoplanets in Kepler’s data. So far, dozens of exoplanets have been found this way, and if you find one, the Kepler team will credit you!
But even with all the hard work put in by Kepler, the majority of exoplanets aren’t found this way. Most planet discoveries are made by ground-based telescopes using what’s called the radial velocity method. The way it works is similar to astrometry. When a star moves, the frequency of the light it emits changes due to the Doppler Effect. Basically, when a star moves toward us, the wavelengths of light get shorter, and when the star moves away from us, the wavelengths get longer. We can measure those changes extremely precisely, and when we see a star move toward us, then away from us, then toward us again, we conclude that there’s a planet orbiting it.
Radial velocity has been extremely productive in finding exoplanets. So far, 456 planets have been identified using this method. It’s also much cheaper, because no large telescopes have to be launched into space. However, it has its drawbacks. It’s only reliable when used on nearby star systems, not all stars can be analyzed this way, and, most importantly, this method only gives a minimum possible mass for an exoplanet. This is because the orbits of some exoplanets might be inclined. We can only see the back and forth movement of the star, and not the up and down movement, so there’s no way of measuring how much the orbit is inclined, so we might view a star as moving less than it actually is.
These are only a few of the possible methods for finding exoplanets, although they are (or will be) the most common. There are plenty of other ways to find exoplanets, using pulsars, gravitational lensing, and all sorts of other things. Sometimes it seems like the methods astronomers use to find exoplanets are as diverse as the planets themselves. That is, right up until we find a planet made entirely of brown rice.
Featured image is a direct image of a possible exoplanet orbiting the star AB Pictorus. Source.