Author: Dr Leila Battison
Since the earliest humans walked the Earth, we’ve turned our eyes skyward. The night’s sky has held mystery and wonder in equal measure for millennia and it’s our sheer curiosity as human beings that has driven our exploration of the cosmos around us. Early study of the skies was philosophical – concerned with divining the exact nature of the heavens – but once it was established that we were on a planet orbiting a star, among countless billions of other stars, then the mysteries resolved into more tangible questions. Are there other planets orbiting those stars? Are we alone?
Modern-day research attempting to address those questions comprises the ongoing search for exoplanets – rocky or gaseous planets that are orbiting stars other than our own. Aside from curiosity, finding such planets would give Earth-bound scientists a clearer idea of how our own planet and solar system were formed. Another major goal of this high-resolution exploration is to find planets that could host life. If a planet is orbiting the right kind of sun at a distance that allows water to stay liquid, rather than evaporating or freezing, then it is said to be in the habitable zone. Earth-sized planets in the habitable zone are thought to be the best candidates for finding extra-terrestrial life forms, so by searching for these planets, we are, by extension, directly searching for life (1). Even without the hope of alien life forms, a habitable planet around a foreign star could potentially become ‘Earth 2’ – a future home for the ever-expanding human population, much anticipated by sci-fi fans.
Perhaps we’re getting ahead of ourselves though. Since our Milky Way galaxy alone contains at least 100 billion stars (2), the nearest of which is 40 trillion kilometers away (3), even spotting exoplanets is no easy task. While stars give off their own light, which travels through the empty cosmos to reach our telescopes and eyes, any planets that are out there are much more elusive. The small amount of light they reflect from their own star is soon dispersed over the vast distances, and overwhelmed by the star itself, so direct imaging becomes almost impossible.
A Flea in the Headlight
Instead, scientists must turn to some more ingenious techniques to detect what are essentially tiny rocks in the great vastness of space. Early planet-hunters used the radial velocity method. As a planet orbits a star, it tugs on it by a small amount, and makes the star itself wobble in place as it completes a whole orbit. Instruments on Earth can detect this wobble as the star moves slightly towards and then away from us, shifting the colour of light that the star seems to give off. When it’s moving away from us, it appears slightly redder than usual, and when it’s moving towards, it seems more blue. Any star that shows this shift potentially has a planet orbiting it, and by watching the star over a prolonged period, scientists can determine the orbital period and the minimum mass of that planet. The technique requires very sensitive spectrometers, but it has been successful in detecting hundreds of exoplanets to date. It’s especially good at finding so-called ‘hot Jupiters’ – very massive planets orbiting very close to their sun – since these produce more noticeable movements of the parent star (4).
While radial velocity has been effective in detecting hundreds of exoplanets, a different technique, called transit photometry, has been responsible for thousands of detections. Again, this involves focusing on the stars themselves, but instead of monitoring the colour of light they give off, it is more concerned with the intensity of light reaching our instruments. When a celestial object passes between our eyes and the sun, it blocks the sun’s light, causing an eclipse. We see an extreme version of this as the moon entirely covers the sun in a total solar eclipse. But the blocking of starlight can be seen at great distances, even when the planetary object doing the blocking appears small relative to the star. These are known as transits and the dimming of the stars’ light intensity curve is proportional to the relative size of the orbiting planet, or planets (5).
Transit photometry is an incredibly sensitive technique and was deemed so promising that it formed the basis of the Kepler mission, launched in 2009. Kepler is essentially a space telescope that stays pointing at the same patch of sky, observing the intensity of light coming from 200,000 stars all at once. By staring unblinkingly at these stars, the instrument is able to detect very short-lived transits as a planet flickers across the face of its sun. Kepler can detect a dimming equivalent to a flea passing in front of a car’s headlight (6).
One of the problems that was encountered by the Kepler telescope was that stars tend to be a lot more flickery than we might imagine, making it difficult to pick out dimming by transits from noise. Although space telescopes are located outside the Earth’s atmosphere to do away with distortion by air particles, there is still an amount of random flicker in the stars Kepler is watching, and that can make transit spotting difficult. An extensive vetting process is needed to discern noise from real clear transit curves, after which around 90% of initially detected dimming signals are discarded (7). Then, to turn many of the suspected exoplanets into confirmed ones, the scientists behind the Kepler mission go through a process called ‘verification by multiplicity’, that uses the balance of probability to detect and confirm multiple planetary systems (8).
Super-Earths, Mini-Neptunes, and billions of habitable worlds
Thanks to these clever technological and statistical innovations, the enduring search for exoplanets has been spectacularly successful. Since the very first confirmed detection in 1992, of two planets orbiting a pulsar, radial velocity and transit photometry have resulted in 3639 planets being confirmed as of August 2017. Among these, the eight-year Kepler mission has been responsible for a remarkable 2335, with a further 1699 waiting for confirmation (9). All from just one small patch of sky. So it seems that, far from being a rare occurrence, planets are a pretty common sight in the cosmos.
By combining observations of size and orbital period, both provided by transit photometry, with calculations of minimum mass from radial velocity measurements, it’s been possible for planet hunters to go beyond simply tallying exoplanets. Around 25% of the confirmed planetary candidates are located in multi-planet systems, just like our own, and we now have a substantial dataset that provides insights into how solar systems typically form. Recently, scientists were surprised to find that there is a clear divide in the size that smaller planets tend to be, which goes against their expectations of a normal distribution. From the Kepler observations, they found that planets either tend to be ‘super-Earths’: rocky planets with a thin atmosphere and a diameter up to 1.75 times that of Earth, or gaseous ‘mini-Neptunes’ with diameters of 2 to 3.5 times that of Earth. Revelations like this have been critical in understanding the formation of seemingly familiar alien worlds (10).
We’ve also made some meaningful progress towards finding habitable worlds. Among the 4000 or so candidate planets found by Kepler, around 50 are roughly Earth-sized and lying within the narrow habitable zone around their star, where water remains liquid; 30 of these have been confirmed. Among these is one of the most promising candidates for an Earth-like environment. Nattily named KOI 7711, it is the closest to ‘Earth 2’ we’ve found yet, with a diameter only 30% wider than Earth’s and an orbit that lasts almost exactly one year (11).
Further, in a bold extrapolation of Kepler’s data, American scientist Erik Petigura suggested that around one in five sun-like stars have Earth-sized planets orbiting them in their habitable zones. That brings the total number of potentially hospitable worlds, and in the Milky Way to around 11 billion (12). It looks like we have quite a lot of exploring to do!
The Search Continues
Before we start up the rockets and pack the freeze-dried ice-cream, though, there’s a problem. Despite our current technology’s remarkable ability to spot tumbling balls of rock hundreds of light years away, we’re really no closer to setting foot on these planets than we ever have been. The manned missions to the moon, and probes we’ve sent to the edge of our solar system are vanishingly small distances on the cosmic scale. It has taken the light from the stars under Kepler’s gaze hundreds of years to travel to Earth at the speed of light. For a spaceship travelling at a fraction of this speed, the journey time would be more like several thousand. Even attempts to communicate with intelligent life forms on one of these distant exoplanets (if they existed, if we could detect them, and if we could understand them), would take hundreds of years for successive responses to be relayed.
So, for now, the search goes on. Kepler’s mission will be over by the end of 2017, but there are already two new projects lined up to replace it. The Transiting Exoplanet Survey Satellite (TESS) will, just like Kepler, stay focused on a patch of sky with hundreds of thousands of stars in the hope of catching a dimming flicker as exoplanets pass by. Meanwhile, the James Webb Space Telescope aims to take a closer look at the atmospheres of alien worlds, in the next step to truly understanding where we, or anyone else, could find a home among the stars (13).