Ever since we first discovered a planet outside our solar system way back in 1988 the hunt has been on to find another planet like our own. That’s not a particularly easy quest however as the methods that we currently have at our disposal, namely the radial velocity method (looking for little wiggles in the parent star) and the transit method (dimming of the parent star from our point of view) are heavily skewed towards finding large planets close to their parent stars. Finding a planet like ours by these methods not only requires precision tools but also great lengths of time, on the order of 2 years or so to ensure that what we saw wasn’t a fluke. Thus a planet like our own has escaped our detection because we’re simply unable to detect it.
The start of this year saw some major progress with the data from NASA’s Kepler mission. Back in January I blogged about Kepler-10b which is the smallest exoplanet that had been discovered to date. Unfortunately for planet hopefuls (and the planet as well) Kepler-10b was found no where near the habitable zone of its parent start. In fact the planet orbits its star in just under a day, putting its orbital distance at around 1/20th of the distance between mercury and our sun. The surface temperature there is enough to melt iron, eliminating the possibility for any kind of life as we know it to arise there.
However the astronomers working on the Kepler data didn’t just stop there as there. Within the wealth of Kepler data are some 48 other earth-like candidates, planets with orbital periods that places them squarely in the habitable zone of their parent stars. One of those potential candidates was Kepler-22b, a planet who’s orbit is much closer in than earth to its parent star which is a lot dimmer than our own. This mean that it was just on the edge of the habitable zone and the last piece of the puzzle was how big it was compared to earth. That last piece of the puzzle was just revealed today and Kepler-22b’s radius is 2.4 times the size of earth.
Whilst that makes Kepler-22b sound like some kind of giant it’s still within the boundaries of what we currently believe to be hospitable to life. The real kicker for Kepler-22b will be finding out its mass as currently, whilst we suspect that it’s rocky like our planet due to its position, we have no idea what its actually comprised of. Right now it could be anything from a planet covered entirely in oceans to a giant rock ball with little to no atmosphere. We can find out the mass by using the radial velocity method and I’m sure that’s the next step that NASA is taking in attempting to figure out just what kind of planet Kepler-22b is.
This might be the first exoplanet to be confirmed as being within the habitable zone but what’s more exciting is the prospect that we have another 48 candidates just waiting for their confirmations to come through. What we can infer from this is that our solar system’s composition isn’t unique and that the formation of terrestrial planets like our own is quite common. That means the conditions that brought about life on our planet are also common in other solar systems which leads to the tantalizing prospect that there’s other life out there just waiting for us to discover it. Of course it will be a long time before we manage to get there and see it for ourselves but it’s still incredibly exciting and I can’t wait to see what other kinds of planets we dredge up from the next 48 potentials.
Humanity, for the longest time, has been aware of planets outside the one that we reside on. Ask anyone today about the planets in our solar system and they’re sure to be able to name at least one other planet but ask them about any outside our solar system and you’re sure to draw a blank look. That’s not their fault however as the discovery of planets outside our solar system (which is by definition, not a planet but an exoplanet) is only recent, dating just over 20 years when the first was discovered in 1988. Since then we’ve discovered well over 500 more planets that exist outside our immediate vicinity and whilst their discovery is great none of them have yet been much like the one we currently call home.
In fact the vast majority of the exoplanets that have been discovered have been massive gas giants orbiting their parent stars at the same distance as Mercury orbits from our sun. This threw scientists initially as back then our current theories on solar system formation didn’t support the notion of large planets forming that close to their parent star. However as time we found more and more examples of such planets, these hot gas giants orbiting at velocities the likes we’d never seen before. The reason behind this is simple, the methods we use to find exoplanets are quite adept at finding these planets and not so much those which we’d consider potential homes.
The method by which the vast majority of exoplanets have been discovered is called the Radial Velocity method. As a planet orbits around its parent star the parent star also moves in tandem, tracing out an elliptical path that’s pinned around the common centre of mass between the two heavenly bodies. As the star does this we can observe changes in the star’s radial velocity, the speed at which the star is moving towards or away from this. Using this data we can then infer the minimum mass, distance and speed required to induce such changes in the planet’s radial velocity which will be the exoplanet itself. This method is prone to finding large planets orbiting close to their parent stars because they will cause larger perturbations in the star’s radial orbit more frequently, allowing us to detect them far more easily.
More recently one of the most productive methods of detecting an exoplanet is the Transit method. This method works by continuously measuring a star’s brightness over a long period of time. When an exoplanet crosses in front of its parent relative to us the star’s apparent brightness drops for the time it is in transit. This of course means that this method is limited to detecting planets and stars whose orbits line up in such a way to cause a transit like this. For earth like exoplanets there’s only a 0.47% chance that such planets will line up just right so we can observe them but thankfully this method can be done on tens of thousands of stars at once, ensuring that we discover at least a few in our search. Exoplanets discovered this way usually require verification by another method before they’re confirmed since there are many things that can cause a dip in a star’s apparent brightness.
There are of course numerous other methods to discover planets outside our solar system but for the most part the vast majority of them have been discovered by one of the two methods mentioned above. For both of them they are heavily skewed towards discovering big planets with short transit times as these produce the most observable effects on their parent stars. Still this does not preclude them from finding exoplanets like earth as shown with the recent discovery of Kepler10-b, a small rocky world in torturous conditions:
The planet, called Kepler-10b, is also the first rocky alien planet to be confirmed by NASA’s Kepler mission using data collected between May 2009 and early January 2010. But, while Kepler-10b is a rocky world, it is not located in the so-called habitable zone – a region in a planetary system where liquid water can potentially exist on the planet’s surface.
“Kepler-10b is the smallest exoplanet discovered to date, and the first unquestionably rocky planet orbiting a star outside our solar system,” said Natalie Batalha, Kepler’s deputy science team leader at NASA’s Ames Research Center in Moffett Field, Calif., at a press conference here at the 217th American Astronomy Society meeting.
Kepler-10b is the smallest transitioning planet to be confirmed to date and shows that it’s possible to discover worlds like our own using current technology. As time goes on and the amount of data increases I’m certain that we’ll eventually find more planets like these, hopefully a bit further out so they’ll be in the habitable zone. The Kepler mission is just a few months shy of its 2 year anniversary with at least another 1.5 years to go and if all goes well it should be returning swaths of data for us for the entire time to come.
I’m always fascinated by the latest discoveries in space even when they’re something like a molten mercury 564 light years away. Our technology is becoming more advanced with every passing day and I know that future missions will end up discovering millions of planets at a time with thousands of potentially life supporting worlds. It’s amazing to think that just 3 decades ago we couldn’t be sure that planets existed outside our solar system and today we know for sure there are more than 500 of them out there.
Ain’t science grand?