Exoplanets - an overview
Not long ago, the only planets known were those in our own solar system. That’s now changed. We’re finding them elsewhere in the galaxy - and these are known as exoplanets. I’m going to walk you through what they are, how we find them, and what it took to get us to where we are now. You could think of this article as a “tapas” serving of what’s been cooking in the field for the last couple of decades.
From where we stand now, it’s estimated there’s a trillion exoplanets just in the Milky Way (our galaxy), based on the Kepler space telescope data. Bringing this down to numbers we can all understand, NASA’s latest figures at the time of writing are that 4,383 exoplanets have been discovered and confirmed - plus another 5,912 candidate planets identified, awaiting verification. (A planet is considered "confirmed" once it is checked through additional observation using two other telescopes.) It appears that there are exoplanets everywhere we look.
The Huate-Provence Observatory, France, used by Didier Queloz (see below) Credit: Jose Rodrigues
The current enthusiasm for finding exoplanets was a long time coming. Institutional resistance, verging on prejudice, dominated the field. Paul Butler, an astronomer at the Carnegie Institution of Science, brilliantly summed it up as follows: “If you went to an astronomy meeting and people asked what you do, you couldn't say you were looking for planets. They would move away from you like you smelled bad.” For extra emphasis, he added: "You might as well be talking about little green men."
The taboo around exoplanets wasn’t entirely baseless. The 20th century was littered with false claims of planets being “discovered” - not helped by flawed telescopes, blatant miscalculations, and articles appearing in high-profile publications that had to be retracted. Despite these difficulties, a few astronomers - willing to stick their necks out - managed to get their discoveries acknowledged in the early 1990s.
The first confirmed exoplanet discovery took place at a ground-based observatory in 1992. Astronomers Aleksander Wolszczan and Dale Frail tuned into a pulsar (a dead star) 2,300 light years away. They’re called pulsars because they emit regular pulses in the form of radio frequencies. Wolszczan and Frail were able to detect planets orbiting this dead star because the pulses were sometimes irregular. We’re talking milliseconds here.
Although this is nowadays acknowledged as an important discovery, the majority of astronomers viewed these pulsar planets as a cosmic anomaly; intriguing, but not necessarily groundbreaking. Someone still needed to prove that planets exist around a “live star”, rather than a dead one.
Didier Queloz, a graduate student at the University of Geneva, made the breakthrough discovery one January night in 1995. Queloz was on the hunt for exoplanets using a system of detection called radial velocity – or, in layman’s terms, looking for stars that wobble. He found one particular star was wobbling ever so slightly. It was exactly what he’d been looking for. The wobble aspect is crucially important because it indicates a gravitational pull on the star from another object relatively nearby, such as a planet. “At this time, I was the only one in the world who knew I had found a planet. I was really scared.”
Queloz was just a student - he wasn’t “meant” to find a planet. Michel Mayor, his supervisor, was obviously aware of the high stakes and potential impact of such a discovery. So, together, they had to be absolutely sure of every aspect of the data. They kept it quiet whilst checking, double-checking, and cross-checking, until they formally announced it at a conference in October 1995. They were both awarded the Nobel Prize in Physics for their discovery - but not until 2019, almost 25 years later.
Astronomers were divided after news of the discovery broke, with prejudicial resistance still lingering. Debra Fischer, Professor of Astronomy at Yale University, said that only “50 percent or fewer believed it was an exoplanet”. Despite this, the discovery allowed the floodgates to open as many enthusiastic astronomers joined in the search.
Discovering exoplanets is not as straightforward as just spotting them through a telescope. Extra equipment is necessary to analyse complex data. Several methods have been developed since the 1990s. Radial velocity - spotting the wobble – is still one of the most successful. However, the transit method has yielded the most results to date. A planet orbiting a star can be detected by measuring the minuscule dip in starlight as the planet passes between us and the star. You can even tell the size of the exoplanet by how much or little light is blocked. There are several other methods of detection as well, though less productive.
The Kepler space telescope (artist’s concept) Credit: NASA/JPL-Caltech
Ground-based observatories have their uses, but searching from space has been a lot more efficient. The Kepler space telescope was launched in March 2009, specifically to discover Earth-sized planets orbiting other stars, and it’s been a successful mission (now ended). The Transiting Exoplanet Survey Satellite (TESS), launched in April 2018 aboard a SpaceX Falcon 9 rocket, was the next major step in the search for planets outside of our solar system - including those that could potentially support life. Now in its extended phase, the mission has already been a “roaring success” according to Patricia Boyd (project scientist at NASA’s Goddard Space Flight Centre). There are others, too, plus exciting space telescopes planned for the near future.
Astronomers are finding a diverse range of planets. They’ve found some much bigger than Jupiter, which is the largest in our own solar system, and many where one side remains in perpetual darkness. Binary systems of two stars have also been discovered, conjuring up images like that of Luke Skywalker watching two suns setting on Tatooine in the original Star Wars film.
The first Earth-size habitable-zone planet discovered by TESS (artist’s illustration) Credit: NASA’s Goddard Space Flight Centre
Scientists aside, what the general public want to know is if there’s life elsewhere. For planets to potentially sustain life, they need to be within what is commonly called the Goldilocks zone. It’s like the “perfect porridge” in the fairy tale of Goldilocks and the Three Bears; neither too hot nor too cold. What makes a planet habitable is largely to do with the temperature being suitable for liquid water to exist on its surface, since water is essential to life as we know it. If a planet is too far away from its star (or stars), any water would be frozen solid. Too close and water would have evaporated away.
The size and distance of the Goldilocks zone depends on the type of star. If it’s a Sun-like star, which is considered mid-range hot, it may be quite far away. But in the case of cooler stars, like orange or red dwarfs, the habitable zone will be a lot nearer to the star.
Estimated numbers of habitable planets in the Milky Way vary a lot, from 300 million to as many as 40 billion. With further study and analysis, astronomers will eventually arrive at a figure closer to the truth. Whatever the actual number, we at least now know that there’s a lot of planets in the galaxy which are suitable for life. As we say on our home page, we’re living in exciting times.
Written by Victoria de las Heras, 7th May 2021