Over the past few months, NASA’s Kepler mission has repeatedly made headlines with announcements of new and unusual planetary systems around other stars. To name a few, we’ve had the most densely packed planetary system known; a planet with two suns; roasting, Earth-like planets; and a giant that might be habitable.
But these headline results have only scratched the surface – there’s far, far more to come.
Birth of a satellite
The Kepler satellite was launched on March 7, 2009, starting a three-and-a-half year mission to search for planets around distant stars.
The telescope is pointed at an area of the northern sky, between the constellations of Cygnus and Lyra, almost in the plane of our galaxy. It stares at that field, continually monitoring the brightness of more than 100,000 stars, sending the data it obtains back to Earth, to be pored over by interested scientists.
The search for exoplanets has already revealed that a significant fraction of all stars host planets.
That means, within the Kepler field, there will be tens of thousands of planetary systems awaiting discovery. Unfortunately, the great majority of those systems will go undetected by Kepler, which watches for the rare blinks in a field star’s brightness that occur when a planet passes between Earth and that star – an event known as a “transit”.
The great bulk of planets move in orbits that do not transit their host star, at least from our viewpoint. Instead, their orbits are tilted, meaning the planet passes above or below the star on the sky, rather than directly through our line of sight.
But if a planet’s orbit is aligned just right, it will transit across its host star once per orbit, causing the star to “wink” periodically.
A single wink does not a planet make
Before Kepler can claim to have found a candidate planet, it must observe at least three consecutive transit events. The first two indicate the orbital period of the potential planet (one transit for each orbit the planet makes around its host star), and the third confirms that the first two transits were from the same object.
One of the key goals of the Kepler mission is to attempt to work out how rare, or how common, Earth-like planets are. The larger the planet, the more of its host star’s light it will obscure during a transit, and so the more obvious the wink will be.
Thanks to its highly sensitive cameras, and the fact the satellite’s view is not obscured by the atmosphere, Kepler is more than capable of detecting planets as small as our own.
Around a sun-like star, a planet such as Earth (just the right distance from the star to host liquid water) would have an orbital period of around a year. So, to be sure we’ll catch three transits, we need a mission of a little bit longer than three years.
Give me more
Kepler has been gathering data for almost three years – and so it’s almost certain it will have observed multiple transits from Earth-like planets that lie just the right distance from their stars to be considered “habitable”. But the vast amount of data being collected by the satellite means results from Kepler are being announced well after the data that lead to them was captured.
A significant amount of work is needed to confirm that a “candidate” planet is really what it seems to be. In fact, Kepler has discovered more than 2,300 potential planets, but only 61 of those have, to date, been confirmed as planets.
As things stand, we’ve really only scratched the surface of what Kepler will discover and there are tantalising hints about future results. The Kepler team has estimated that 207 of their 2,326 candidates are comparable in size to Earth, and almost 50 of those are within their star’s habitable zone.
Given these results are entirely based on the first year of Kepler data (and the satellite’s now been up there for three) it’s fair to expect those numbers to rise dramatically as more data is analysed. There will definitely be lots of exciting Kepler discoveries to come in the next few years.
Once the data from the main Kepler mission are analysed, we’ll no longer have to speculate as to whether the Earth is unique. We won’t know whether those planets have oceans, or life, but we will know how often planets form that are roughly the same size as ours, and in roughly the same place.
In addition it’ll be possible to determine the distribution of planets as a function of distance from their star for planets ranging in size from similar to Earth to bigger than Jupiter.
A huge wealth of related scientific results are also coming out of the mission. By surveying so many stars non-stop, Kepler is discovering vast numbers of eclipsing binary stars, and yielding unprecedented information about how stars vary and pulsate – results that will trigger vast improvements in our understanding of how stars actually work.
Keeping Kepler alive
As things stand, funding for Kepler is scheduled to run out towards the end of this year, at the end of its originally allotted lifespan. The Kepler team would like the mission to be extended to run for at least eight years – which is technically feasible – but whether the US Congress will find the funds to support it is still unclear.
Members of the Kepler team aren’t the only people finding planets using data sent back from the satellite. For the past 15 months or so, it’s been possible for anyone to participate in the analysis of Kepler data.
The Planet Hunters website aims to make searching for planets in the Kepler data fun and accessible, and is using the keen eyes of volunteers to search for transits that the Kepler team and their powerful analysis tools have missed.
There have already been numerous success stories, such as new candidate planets being found that had previously been missed. The site has proved a remarkable success.
I find it truly remarkable that, less than 20 years after the discovery of the first planets orbiting sun-like stars, amateur astronomers and members of the general public are able to participate in the detection of other Earths.
Long may such work continue.