Recently there have been some news on new methods for exoplanet searching as well as improvements on current methods. Searching for planets outside our solar system is one of the newest and most exiting fields of astronomy, so I thought I’d go and make me a bit of an overview of what is hot in the field.

First thing’s first

So far, 287 planets have been found orbiting stars in the galactic neighborhood. That’s 287 more than were confirmed just 13 years ago, and the field is rapidly expanding to make this number a lot larger in the next couple of years as technology enables us to detect smaller planets in larger orbits, as well as performing large surveys of the sky. The thing about almost all of the exoplanets we know today is that they’re quite strange compared to the planets in our own solar system, and compared to our understanding of planet formation. We have small, rocky planets close to the Sun, and larger, gassy giants in the outer parts, and what we’ve found is a bunch of supergiant planets extremely close to their star. Could our solar system really be so special? The most successful method to date for finding exoplanets, the method of radial velocity, is currently only capable of finding these massive planets that orbit close to their star, so it’s not so strange that we only find those you might say. However, if we understand just a little about planet formation, these systems should be VERY rare, so just maybe this is only the tip of the iceberg, the statistical exceptions, and if we just look a little closer we will be able to see a lot of planets that look just like our Earth.

That’s exactly what a world of planet hunting astronomers have set out to find, and technology has to be pushed pretty far for them to succeed.

Combing the galaxy

Starting with the beloved radial velocity method, scientists have recently proposed a method for improving the accuracy of this method around 60 times. The radial velocity method uses the fact that the planet doesn’t really orbit the star, but, just like the star, orbits the center of mass of the combined system of planet and star. This means that the star will have a velocity away from or towards us (unless the plane is exactly perpendicular to ours of course), and because of the Doppler shift of light waves (think of the shift in pitch of the sound from passing ambulances), we’re able to determine all sorts of things about the planet or planets around the star. But the precision of the measurement has only been good enough to measure larger planets in smaller orbits, simply because they are rocking the star the most.
The new method, that uses a so-called laser comb, increases the precision to the point where earth-size planets in earth-like orbits could be detected. This is of course nice because it will enable us to find planets where life as we know it can exist, but it is also necessary for obtaining a larger number of discovered planets, making better statistics about planets possible. With better statistical data we’ll be able to create better theories for planet formation and distribution.

Transits

Another successful method for exoplanet hunting is the transit method. With this method we look for planets that are passing in front of their star by measuring the brightness of it. If the brightness of the star takes a characteristic dip, it could mean that an exoplanet is passing in front of it. We can then continue observing it and use other methods, like the radial velocity method, to confirm the existence of the planet and determine important physical properties. The main disadvantage with this method is of course that the planet has to be exactly in front of the star in our line of sight, and the probability of this can be very small, especially for planets in larger orbit. But unlike the radial velocity method mentioned above, where observations have to be done over weeks or months, this method has the advantage that transits can be seen on the light curve when they happen. This introduces the possibility of surveys that looks for transit events on a big number of stars in a small amount of time.

So despite the low probability of transit events, a lot of planets can be found with this method by making surveys of the night sky, and this is exactly what the new SuperWASP (Super Wide Area Search for Planets) collaboration is doing. In recent news they announced the discovery of 10 new exoplanets in only six months, and is so far the most successful transit survey by discovering 15 of the 45 exoplanets that have been found using transits. Currently it will not find planets much smaller than Jupiter, the largest planet in our solar system, but it can lead the way for future space-based missions, like the Kepler Mission, that will be able to find Earth-like planets and give us some really nice statistical data on planets in our galaxy.

Microlensing

In the theory of General Relativity Einstein showed that light is affected by gravity, just like normal matter. So if a distant light source passes behind a heavy object in our line of sight, we will see the source as if it was passed through an optical lens, and we call it gravitational lensing. If the lens has a mass comparable to that of a star, it is called microlensing, and this is among other things used to search for planets in the Galaxy. The light will be magnified in a special way when the planet is passing in front of the background source, so this method is also based on observations of the light curve.

These lensing events are very rare, and it is difficult to locate the planetary system again after a lensing event. The method is thus best suited for large surveys and statistics rather than detailed investigation, simply because we usually only have one chance. The advantage is however that the microlensing method works well for smaller planets in larger orbits, and can thus be used to detect habitable planets already. Of the 8 observed microlensing planets, the record is so far a mass of around 5,5 times that of the Earth. To give it a nice and memorable name, this planet is called OGLE-2005-BLG-390Lb. Future space-based missions, for example using the SIM PlanetQuest satellite, will take the surveys to a new level and provide much more data than can be currently obtained.

Looking ahead

While all of these clever ways of finding planets around other stars are pretty cool, the most exciting method of planet hunting is also the simplest to understand. Namely looking at the planets directly with a giant telescope. While this sound pretty simple, there is a slight problem resolving the light from a planet millions of times less bright than its star. Its like being able to distinguish a firefly next to a light house at a huge distance. If however seen directly, we will be able to determine a lot of the properties like surface temperature, atmospheric composition etc., that are all very important for investigating the possibility of life on the planet.

One planet has so far been seen this way, but space-based missions like the Terrestrial Planet Finder, the Darwin Mission and enormous ground based telescopes like the Extremely large telescope would be able to do this easily. Unfortunately, all of these are in very early stages of planning, and are being subjected to budget cuts and all the usual stuff.

But the next couple of years are going to be awesome with regards to exoplanet hunting. A lot of new, promising, methods are being developed and used to give us better insight into planet distribution and formation and possibly one of the most tantalizing subjects: life in the Universe. My personal guess is that we will see the discovery of the first Earth-sized planet within 5 years, and who knows when we’ll find the first Earth-like planet with a thick, oxygen-rich atmosphere and liquid water? Let’s get going.