Planet Hunt

What is an Exoplanet?

Dusty Disk around Beta Pictoris

An exoplanet, or extra-solar planet, is a distant planet that orbits around a star other than our Sun.

Up until the 1990s, the only real evidence we had that planets might exist around other stars was the discovery of dusty, rocky disks of material surrounding nearby stars, such as Beta Pictoris in the right-hand image. Then, in 1995, came the discovery of a planet around the sun-like star 51 Pegasi. Since then, astronomers have used various methods to discover more than 200 new exoplanets scattered around local regions of the Milky Way.

One strange and unexpected fact about the 51 Pegasi planet, is that it has an incredibly short orbital period; so much so that it only takes 3.5 days for the planet to go right around its parent star. Physics tells us that planets with shorter orbital periods are closer to the central star, whereas those with longer periods are further out. Our own planetary system sees Mercury, the closest planet, with an 88 day orbital period and the dwarf planet Pluto, with a 248 year period.

Now surface temperatures on Mercury are hot enough at around 400 °C, but for a planet with a 3.5 day orbit they are expected to be well in excess of 1200 °C. It's fair to say that we do not expect to find life on such a planet. In addition, it has been calculated that the mass of the 51 Pegasi planet is similar to that of Jupiter. In fact, the suggestion from experts is that it's a large gas-giant planet just like Jupiter - a 'Hot Jupiter' so to speak.

Of course we cannot be sure about planet type (rock or gas) because we can't actually see the planet. In fact exoplanets are so far away from us that we haven't actually seen any of the 200+ we know about - so how do we know they are there!

The vast majority of exoplanets (including 51 Pegasi b) were discovered indirectly through the gravitational influence they exert on their parent star. In other words they make their host stars 'wobble' about as the star-planet system circles around a common centre-of-mass. Take a look at the animation below to see this in action.

Exoplanet Detection Methods

Exoplanets cause their parent stars to wobble

If we look along the plane of the planet's orbit, we see the wobble as movement of the star towards and away from us. If we are looking down on the system from above, as in the animation, then we see the wobble as an astrometric shift, i.e. the star does a small circle in the sky when compared to other nearby stars which are fixed in position.

However, astronomers find it much easier to spot motion toward and away from us, i.e. the star's radial velocity, by observing its spectrum. Current technology allows us to detect radial velocities of just 1 metre per second - a fast walking pace. Jupiter causes the Sun to wobble by up to 12.5 metres per second, so it's no surprise that astronomers are now finding planets in Jupiter-like orbits.

There is of course another method of finding exoplanets and one which is simple enough that we can do it with the Liverpool telescope - hence this project. The method is based on looking for regular dips in the brightness of a star as a large planet passes, or transits, in front of it.

Planetary Transits

For close-orbiting giant planets, this dip in brightness can be as much as 2% of the original brightness and occurs every couple of days. In the wobble animation (above), the planet is orbiting at right-angles to us. As a result, there is no chance of it passing in front of the star, but we know of at least 14 cases, including TrES-1, where transits do occur.

In fact, given the millions of stars in our galaxy, the chances are that many more are orientated such that a planet will block out a fraction of the host star's light at some point. It's thought that around 1% of stars might have a transiting planet, although it is difficult to catch one in the act.

Animation showing the drop in the light-curve as a planet transits its parent star.

© Hans Deeg

So what can we learn from a transit?

Well, the first thing we learn is the inclination of the system, which must be close to 90°. Note that 90° refers to looking at the system edge-on and 0° is looking face-on, as in the wobble animation. We can also measure the orbital period. If we then combine this knowledge with the results of the 'star wobble' method, we can calculate the orbital velocity, planet mass and orbital distance. Finally, by looking at the shape of the star's light-curve we can make a reasonable estimate of the size of the planet.

The Future

Searching for planetary transits will soon become the primary method for detecting exoplanets. It is true that the 'star wobble' method has found the vast majority of exoplanets but it's thought that most of the exoplanets that can be found by this method already have been. The transit method is less restricted by technology and is generally believed to be the way ahead. So much so that there are currently over twenty groups around the world looking for transits, and some of these plan to use the Liverpool telescope .... just like us!

Anyway ... hopefully that gives a bit of background for the experiment we are about to undertake. By studying the light from the TrES-1, we hope to repeat the detection of the large planet that transits the star every 3.03 days.