THE HUNT FOR EXOPLANETS

Meet the scientists who are hunting for worlds outside our Solar System in an attempt to find Earth-like planets that, like our own, could be hospitable to life.

Written by Jonathan O’Callaghan

Up until 1992 the thought that there might be other sentient life out there in the universe was one that was generally met with incredulity and disbelief. Those branding aliens as real were looked upon disdainfully, and the mere suggestion that humans were just one of many types of life in the cosmos was not one that was given much credibility.

However, the discovery of an exoplanet (one outside our Solar System) orbiting the pulsar star PST B1257-12 in 1992 changed our entire preconception of the universe. If these planets could exist around such a volatile star, then surely they could also be found around more serene stars and especially those like our own Sun, where we already know the eight planets of our Solar System exist. And of course we also know one planet in our Solar System, Earth, is capable of supporting life. Extrapolating that ratio, there could be billions of Earth-like planets in our Milky Way alone.

The astronomers who found that first planet were Dr Dale Frail and Dr Aleksander Wolszczan. “We didn’t begin by looking for planets”, said Dr Frail. “We just found this unusual pulsar that was behaving in ways we didn’t understand. It turned out to be the most robust planet discovery at the time. I remember very distinctly in 1992 that the planet-hunting community was thrilles about our pulsar planet discovery, because it really told them that they were on the right track, they just need to keep persisting. I think the field was struggling both financially and technically at the time, but the attitude after our discovery was that if planets can form around pulsars then they could form around anything. We gave a real boost to that community.”

And so, the hunt was on. Where previously planet hunters had struggled to gain funding, the search for exoplanets shot to the top of agendas throughout the world. Agencies from NASA to Roscosmos to ESA began designing and developing planet-hunting telescopes, while private observatories began to devise techniques that would enable astronomers to search for planets around distant stars.

The initial favoured method that was used to find planets was the radial velocity technique, which observes perturbations in a distant star to indicate the presence of a planet. The current preferred way to find planets, however, is the transit method. To date thousands of planetary candidates that we know of have been found using the transit method, which we’ll explain more about later.

But while telescopes around the world trained their sights on potential planet-bearing stars, finding new worlds was proving rather difficult. Up until 2009 barely a hundred planets had been found and confirmed and, owing to the primitive methods available to find them, most of these were giant hot Jupiters with close orbits around their stars – worlds that would be inhospitable to life as we know it.

Finding large planets of this type is much easier because the methods used rely on an observational change in the host star, whether it’s a gravitational wobble or a dip in its light as the planet passes in front of it relative to us. A large planet orbiting at high speed will produce frequent and noticeable effects on the star, whereas a smaller planet in a slower orbit further out is much harder to detect. To find smaller planets, and therefore ones that are more similar to Earth, astronomers needed a way to watch thousands of stars simultaneously rather than just focusing on a few at a time.

Fortunately, NASA had recognised the need for a wide-field observation telescope and had begun working on one at the beginning of the 21st Century. The telescope would be groundbreaking and do something never attempted before. It would be put into an orbit around the Sun, away from any interference, and it would train its sights on a specific area of the Milky Way galaxy. Known as the Kepler space telescope, NASA’s newest creation would prove to be the most important and useful planet-hunting telescope to date.

The Kepler telescope launched in 2009 to its predetermined position 150 million kilometres (93 million miles) from Earth in orbit around the Sun. It uses the aforementioned transit method to find new planets. To understand how it works, imagine you were staring at 20 light bulbs in a grid, and you knew some of these light bulbs had moths flying around them but you weren’t sure which. You observe the bulbs for a period of time and each time one of the bulbs dims you can presume that something has passed in front of it. By observing the dip in the light three times and measuring the time taken for the dips to occur, you can confirm that there is something flying around the bulb and you can work out the speed the object is moving at. With just this data alone, you can even measure the dip in brightness and, coupled with the knowledge of its orbital period and the size of the bulb, you can deduce the size of the object. From just these three calculations you know the speed of the moth, its size and its distance from the bulb.

Kepler does this for 100,000 stars. So it observes all of them simultaneously, watching out for dips in their brightness, and then waits to confirm the regularity of the dips. By doing so it can deduce if there is a planet present around the star and then calculate its speed, size and distance from the star. At the time of writing, the Kepler space telescope is currently out of operation after losing functionality in one of the reaction wheels that allows it to accurately focus on these distant stars, but regardless, since it was launched it has provided scientists with a multitude of data to find new planets, much of which will take another two years to analyse. The field of planet hunting has been kick-started by Kepler with numerous projects now in operation around the world to find planets.

The detection of exoplanets has proved beyond reasonable doubt that almost every star in the universe plays host to a planet of some sort. However, as mentioned earlier the majority of planets we’ve found so far have been large Jupiter-sized worlds, many orbiting their stars in tight orbits and therefore having a scorchingly hot temperature. The holy grail of planet hunting is to find a world exactly like Earth, with the same size and mass in an orbit in its star’s habitable zone, where conditions are just right – not too hot and not too cold – and therefore water will be able to form. To date, no such world has been found.

However, in January 2013 astronomers at the Harvard-Smithsonian Center for Astrophysics reported that there were likely to be at least 17 billion Earth-sized worlds in our Milky Way galaxy alone. When you consider that there are hundreds of billions of galaxies in the universe, it is therefore highly unlikely that only one of these, our own Earth, is able to play host to life.

One of the difficulties in finding Earth-like worlds is the limiting factors of the methods we currently use to find planets. The transit method, for example, relies on multiple observations of the orbit of a planet around a star. Consider looking at our Sun from outside the Solar System; to definitively detect the Earth you would have to observe three transits of the Earth on the Sun, which would take three years. Therefore, only by observing stars for a long period of time can planets in habitable orbits be found. The Kepler telescope has so far completed over four years of observations, so it is hoped that within its data may be hiding some of these Earth-like worlds. It’s also thought that some habitable planets might reside in closer orbits around red dwarfs, which would mean their orbits are slightly faster and therefore detection might be easier.

Perhaps, though, to increase our chances of finding a new Earth we need to change our methods of finding planets. One of the most promising techniques that has already been tested, and will be used more in future, is directly imaging exoplanets. Some modern observatories such as the European Southern Observatory’s (ESO) Very Large Telescope (VLT) have already been able to take images of planets, and future telescopes like NASA’s James Webb Space Telescope and the European Extremely Large Telescope will be able to continue this work.

To directly image a planet, the light of its host star must be blocked out using something called a coronagraph. This allows observations of the area around a star to be made, and any planets in orbit will be somewhat visible. Understandably, though, the method is incredibly difficult. “Smaller telescopes don’t have a good enough angular resolution, so they don’t have good enough imaging precision to really use a coronagraph,” said Dr Neil Gehrels, one of the scientists on the WFIRST programme. WFIRST is a telescope that has recently received funding from NASA with a view to a launch in 2023. It will search for exoplanets using both the microlensing technique and possibly by directly imaging them.

“We’ll find thousands of exoplanets using the microlensing technique, and even some that are in the habitable zone, similar to Earth-like planets,” explained Dr Gehrels. “But then with the coronagraph, the really exciting thing we can do is to make a direct image of an exoplanet. We might be able to get close in to Earth-mass worlds. The telescope has spectroscopy on board, so we could look at the constituents in the atmosphere.”

As Dr Gehrels mentions, studying the atmospheres of exoplanets is another area of planet hunting that has garnered a lot of interest recently. By analysing the light of a star as it passes through a planet and studying the changes in its spectroscopy, scientists are able to determine what sort of atmosphere the planet might have. Some telescopes like the aforementioned James Webb Space Telescope may even be able to make direct analysis of alien atmospheres, finding out if they bear any similarity to ours on Earth and, ultimately, possibly indicating that the planet is habitable.

Every month we seem to hear of an exciting new exoplanet discovery, and as more and more telescopes come online we will continue to find amazing new worlds that are not only similar to Earth but are in fact so dissimilar that they question our current theories of planet structure and formation. Thanks to telescopes like Kepler scientists have a great starting point to search for planets. They already know of thousands of stars with planets in our Milky Way, and as our methods of detection and analysis improve we will move closer and closer to finding a world like our own.

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