So last time we looked at the discovery of the first Earth-size planet by Kepler. In this post I wanted to address the science that goes into discovering planets outside of our solar system, as well as to advertise a really cool Exoplanet App, developed by Hanno Rein (an astrophysicist at the Institute of Advanced Study at Princeton), that is a must have for anybody interested in keeping track of how many exoplanets we’ve discovered. If you have a little bit more time at home and want to play around with the exoplanet website, you can generate correlation plots that have a lot of neat options.
The topic of exoplanet detection first caught my interest in an Astrophysics class taught by Geoff Marcy while at Berkeley. Marcy has discovered a sizable fraction of the first exoplanets discovered, and his elucidation of the subject was enough to leave me enamored by the topic. Most importantly, I want to make it clear to everyone that directly imaging an exoplanet is incredibly difficult, so the vast majority of planets have been discovered by much more interesting, and ingenious, methods. Only a very small handful of planets (14 as of 1/15/2011) have actually been imaged, but this number will increase as the next generation of space telescopes is deployed by Astronomers in the coming decades.
The first and most ubiquitous method of detection is that of Doppler spectroscopy. It turns out that planets don’t actually orbit around their parent star; in reality both the star and the planet orbit around a common ‘center of mass.’ The star’s orbital radius is MUCH smaller than that of the planet’s due to the fact that the mass of the star is MUCH larger. In most cases the orbital radius is smaller than the stellar radius, hence the star looks like it is simply wobbling about, as opposed to orbiting around, a fixed point. Now here is where the heart of the argument lies. As the star wobbles, sometimes it is moving away from us, and sometimes it is moving towards us. When the star is moving away from us, the light that it emits is slightly Doppler red-shifted, whereas when the star is moving towards us the light is blue-shifted. This effect is quite familiar in the context of sound waves for anyone who has listened to a fire engine’s siren as it passes by you.
An astronomer can measure this Doppler shift over many observations and determine the velocity of the star. Typical data looks something like this:
Then, by simply applying Newton’s Laws and Gravitation, the orbital radius and mass of the planet can be determined (well, there are a few complications involving the orbital inclination of the system along the line of sight).
The next type of detection method used by astronomers in their search for exoplanets is that of transits. A transit is the name we give to the event when a planet passes in front of its host star. Transits happen in our own Solar System when Mercury or Venus pass in front of the Sun. They are rare events, due to the very specific geometry that is required for the event. The next time Venus passes in front of the Sun will be on June 6th, 2012. I hope that everyone takes an interest in this beautiful event, since the next time it will happen will be in 2117. Below is an image of what the last transit in 2004 looked like:
Now transits around other stars are nowhere near as detailed since no telescope yet made has the ability to resolve a star. Nonetheless, that little shadow that you see above does have an observable consequence: the light emitted by a star is slightly dimmed during the event by about .01% to 1%. As tiny as this fraction is, it is large enough for astronomers to detect! It always amazes me to know that we have developed technology capable of detecting these incredibly small signals. Typical data for such an event is displayed below:
Fortunately transits around other stars aren’t rare like the ones taking place in our Solar System, and occur regularly enough for Astronomers to measure several of them over the course of many observations. This data, along with, once again, gravitational physics, allows us to determine the properties of the exoplanet. The only drawback to this method is that it can only be used to detect planets whose orbits are aligned along our line of sight. Hence a much smaller fraction of exoplanets have been discovered via this method as opposed to Doppler spectroscopy.
The next method of detection is rare but fascinating: microlensing. This method of detection goes beyond classical Newtonian Physics, and relies on Einstein’s theory of General Relativity. A basic result of GR is that gravitational fields, which are produced by masses, have the ability to bend light, much like a piece of glass called a lens bends light in a telescope. So a massive object floating around in space has the ability to bend the light from objects behind it. This method was first used in the analysis of much larger objects: clusters of galaxies. These giant metropolises of galaxies bend the light of galaxies far behind them creating beautiful images as seen below:
The amount of lensing tells us about the mass of the lensing cluster. In fact, this method is one of the ways in which Astronomers study the mysterious substance known as Dark Matter, since the luminous matter is never enough to create the amount of lensing that is observed. Now microlensing is a much more subtle effect that can occur under very special circumstances. When a single star passes in front of another star, it bends the latter’s light just a wee bit. If there happens to be a planet around the star then it too will pass in front of the lensed star causing a secondary, even more minute, lensing event. Using GR, the properties of the planet can then be determined. This is, pardon my battlestar, frackin’ amazing. Typical data for such an event are shown below:
The final method that has led to confirmed exoplanet discoveries is Pulsar Timing. Unfortunately this method can only be used to find planets orbiting Pulsars, which are quite rare. Pulsars are stellar corpses; tiny rotating neutron stars that result from the collapse of very massive stars at the end of their lives. Due to conservation of angular momentum, as massive stars collapse, they spin faster and faster. Since the star’s magnetic field is typically not aligned with its axis of rotation, the resulting neutron star acts like a lighthouse beacon, pulsing whenever its magnetic pole is aligned with our line of sight.
These massive, compact objects are also the most accurate natural clocks we know of. The ‘pulses’ measured coming from them are spaced out very precisely and do not deviate to a very high level of precision. When a planets orbits a pulsar, the tugs on the stellar remnant alter these pulses in a very specific way. It is within these variations in the timing of the pulsar that an exoplanet can be found.
Well, that about wraps it up. Doppler spectroscopy, transits, microlensing, and pulsar timing are the main methods which have been developed to discover exoplanets outside of the Solar System. Through the application of physics to precise observations, astronomers have been able to find 518 exoplanets as of January 2011 and this number will continue to increase as Kepler continues to look out into the depths of space. With each new discovery we learn a little bit more about our galactic neighborhood, creating a map much like the explorers of Earth did centuries ago. I am truly excited to be living in this day and age, during a time in which we push the frontiers of our knowledge, and refine our understanding of our place in this vast, and beautiful universe. I hope you do too.