Astronomers have discovered more than 3700 exoplanets, or planets outside of our solar system, including over 600 multi-planet systems, with another nearly 5,000 candidate dedications awaiting confirmation. These exoplanet discoveries include massive planets similar to our Jupiter, smaller Earth-sized planets, and even a solar system home to 7 rocky planets similar in density to Earth, Venus, and Mars.
Many of these discoveries owe thanks to NASA’s Kepler mission and its successor K2, a project surveying our galactic neighborhood for small, Earth-sized planets using a dedicated space-based telescope. NASA’s next major exoplanet hunting effort, called TESS, short for Transiting Exoplanet Survey Satellite, will survey over 200,000 nearby stars and is expected to find thousands of new worlds. TESS is set to launch sometime after April 2018 on the SpaceX Falcon 9 rocket.
So how do astronomers find exoplanets, and how much do we actually know about these worlds outside of our solar system?
Transiting Exoplanets
The method used by both the Kepler and TESS space-based missions, and thus the method that has led to the most exoplanet discoveries, is the transit method of exoplanet detection. When a planet passes in between us and a star, the planet is said to be transiting. In our own solar system, we can view transits of Mercury and Venus when their orbits bring them in between us here on Earth and our Sun.
Even though planets are small relative to stars, when the planet’s path moves across our view of its host star, the planet blocks a small amount of light coming from that star. Astronomers can measure the dip in the brightness of the light coming from the star when the planet is in front of it as well as the length of time over which the dip in brightness is observed. Knowing the mass of the star, and with a little help from Kepler’s Third Law of planetary motion, these two measurements can tell you how far away the planet is from its host star. The size of the planet’s orbit is key for getting a first estimate of whether the temperature there might be suitable for liquid water and thus life. Astronomers can also deduce the size of the planet itself if they have an estimate of the host star’s size.
Since we do not know beforehand which stars have planets around them and also which stars have the perfect alignment so those planets will pass in between us and their host star as they orbit, nor can we predict when these transits might happen, using the transit method to find exoplanets requires continuous monitoring of hundreds of thousands of stars. Thus, NASA has invested in dedicated space telescopes like TESS and Kepler that devote their time to this kind of monitoring.
Radial Velocity Method
Even though the transit method for finding exoplanets is more commonplace now thanks to these planet-hunting space missions, the first surge in exoplanet detections were actually made using a different technique known as radial velocity, or the Doppler method. To understand this method, we first have to understand a little about planetary orbits.
We think of planets as orbiting their host stars, but actually the star and the planet together orbit their shared center of mass. When you have two orbiting objects of equal mass, their center of mass will be at the midpoint between them so they will appear to chase each other in their orbits around a central point. Stars, however, are much more massive than planets, so the center of mass in such a system is much, much closer to the host star. This imbalance results in a large, extended orbit for the planet and only a tiny wobble in the host star’s position. Although tiny, this wobble is still measureable.
Astronomers monitor the positions of stars using spectroscopy by looking for a Doppler shift in their emission lines which indicates forward or backward motion relative to us, the viewers. Between 600-700 exoplanets have been discovered this way and it is still the easiest technique to use from ground-based observatories. However, the more massive the planet and the closer that planet is to its host star, the bigger its gravitational influence on its host star, i.e. the bigger the star’s wobble. The Doppler method thus is better suited for finding massive planets very close to their host stars, a type of planet thus nicknamed “Hot Jupiters.”
Direct Imaging of Exoplanets
You may be wondering why astronomers use these indirect methods for finding exoplanets rather than simply taking pictures of them directly. Well, since planets do not emit their own (optical) light and instead only reflect light from their host star, and since they are far smaller in size than their host star, looking for the light of a planet is akin to trying to pick out a firefly hanging out next to a search light. From thousands of miles away.
However, now that astronomers know there are thousands of exoplanets out there waiting to be discovered, they are developing techniques that enable direct imaging by somehow masking the light from the host star. This blocking can be done before or after the light enters the telescope. The direct imaging method is still in its beginnings but it shows substantial promise and around 40 exoplanets have already been found using this method. Direct images have the potential to tell us more about the planets, including information on their atmospheres and compositions, than we can learn from more indirect methods and is thus an important area of active exoplanet research.
The search for exoplanets is driven by our desire to understand how common or unique our tiny corner of existence is and ultimately if we are alone in the universe.
Other Exoplanet Detection Methods
There are a few other, although less common, ways of finding exoplanets, including gravitational microlensing. Einstein’s theory of General Relativity tells us that light traveling through a gravitational field will appear to bend around the massive object producing that field, an effect known as lensing, or in the case of very low mass objects like planets, microlensing. Thus, it is possible to see light from a distant star bend around a planet in the foreground as that planet passes between us and the distant star. The star’s light also gets amplified in the process, so observing this brief spike in a star’s brightness reveals not only that an object has passed in front of the star, but also the mass of that traveling object. Unfortunately, these microlensing events usually are not repeated and so cannot be easily verified.
The first exoplanet ever discovered was found by an entirely different technique known as pulsar timing. Pulsars, or rapidly rotating neutron stars that send out beamed emission similar to the search light on a lighthouse, are incredibly precise in the timing of their rotation. They are so precise and predictable, in fact, that even the tiny wobble produced by the gravitational influence of an orbiting planet will cause a big enough change in the timing of their passing strobe light for astronomers to detect. Although the first exoplanet was found using this technique, the harsh conditions around a pulsar mean such planets would not be suitable hosts for life like us.
The search for exoplanets is driven by our desire to understand how common or unique our tiny corner of existence is and ultimately if we are alone in the universe. Astronomers have used these current techniques to identify exoplanets and can compare these planets to our Earth to first order them by determining the planets’ sizes as well as whether or not the planet is in the star’s habitable zone. However, much more information is needed to determine habitability. For example, here on Earth, we would not survive without our atmosphere, and our efforts to understand the atmospheres around exoplanets are only just beginning.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Image courtesy of nasa.gov
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