Humans have found thousands of planets outside of our solar system in the last two decades or so, but – contrary to many headlines you might have seen – as far as we know, not a single one of them is anything like Earth.
“The bottom line is that when you see these announcements about ‘habitable’ planets, most of the time we don’t know that at all,” Giovanna Tinetti, a professor and exoplanet researcher at University College London, tells BuzzFeed News. “They’re all candidates – but we need to know more to say whether that is the case or not.”
Tinetti is one of a cohort of British scientists trying to take us closer to locating other worlds like our own. She started out as a particle physicist but switched to exoplanets at the end of her PhD. She says it felt like a gamble moving into such a new field. “But it turned out to be a good bet.”
We’ve found plenty of planets outside our solar system. The very first – actually two planets orbiting around a dense, fast-rotating star known as a pulsar – was discovered in 1992. Three years later, the first planet orbiting a sun-like star was found.
"When you see these announcements about ‘habitable’ planets, most of the time we don’t know that at all"
In the years that followed other discoveries trickled in, until NASA’s planet-hunting space telescope Kepler got up into low Earth orbit in 2009 and opened the floodgates. At the time of writing the Extrasolar Planets Encyclopaedia – an ever-growing list of all the alien worlds found so far – contains 3,651 planets.
But it still hasn’t quite managed to provide the one thing people really want: another planet just like Earth, that could be habitable for life as we know it.
A new European space observatory that will launch in 2024 plans to change that. In June this year the Plato mission was given the official go-ahead by the European Space Agency. Its science team, which is being run from the University of Warwick, includes British scientists who will be looking specifically for Earth-sized planets around sun-like stars. Plato will be able to tell us more about those planets and their stars than any mission has to date.
Astronomers are keen to know how unusual (or not) our planet is in the grand scheme of the universe. Another Earth would give us something to compare our own world with, as well as somewhere to focus our search for extraterrestrial life, and maybe even somewhere to dream of moving to and setting up home on someday.
“Let's face it, this is one of the big questions,” Don Pollacco, an exoplanet researcher at Warwick who’s leading the Plato mission’s science side, tells BuzzFeed News. “It's the question that will lead to finding life like ours, if there is any.”
To find another Earth, you need a few ingredients. The first thing to look for is a rocky planet of a similar size and mass to our planet, that orbits at roughly the right distance around a sun-like star. It might be nice if it were the same age as the Earth, too, but those first details are the really important ones.
Its orbit needs to be in what’s called the habitable zone, the exact location of which varies depending on the star. It’s the distance away from each star where it’s believed the temperature would be right for water to exist as a liquid, if there’s any on the planet in the first place. Humans need water to exist, so traditionally, water has been used as a starting point when looking for life outside of Earth.
“Let's face it, this is one of the big questions"
The first planet NASA’s Kepler telescope found in a habitable zone was Kepler-22b. A year lasts 288 days on Kepler-22b, and the planet is relatively close in size to Earth – roughly twice as big. So far, so promising. But with observations from the telescope alone, astronomers couldn’t measure its mass, which we need along with a planet’s size to establish if it’s rocky or made of gas. Ground-based telescopes haven’t been able to work this out, either – which means we don’t know what the planet is made of and, ultimately, if it is like Earth.
It’s one of so many cases in which we’ve been close, but not quite there. “The work that's been done with Kepler shows that you could have something one and a half [times] or twice the size of Earth and it could be a gassy planet,” says Pollacco.
Pollacco has been involved with the hunt for exoplanets for over a decade, first with the UK’s WASP (Wide Angle Search for Planets) ground-based observatories, and now with Plato.
He recalls a meeting that happened several years ago where he realised it could soon be possible to find Earth-like planets with Plato: “To be in a position that before my retirement we can really have Earth 2.0s kicking about, I thought that was exciting – I still do.
“Plato-1 [the first planet the Plato mission finds] will be a planet like the Earth around a star like the sun in the habitable zone. And nobody’s going to be able to turn around in a few years’ time and say ‘Oh, it's full of gas’ or whatever.”
Now it’s been given the green light, work on building Plato will finally begin. It’ll be made up of 34 small telescopes and cameras that will be launched into space and work together to monitor nearby stars.
Plato will work in the same way as Kepler in that it’ll look for planets that pass in front of their stars from our point of view. Not all planets do this. Imagine looking at the solar system from below – you’d see the sun in the middle, with the planets orbiting around it, and none of them would pass in front of the sun from your perspective.
When we do see a planet pass in front of its star, we can use that observation to work out the planet’s size in a technique known as the transit method. It’s proven to be the best way of finding large numbers of alien worlds – and the only way we have so far to measure a planet’s radius. The dip in the light we see relates to how much of its star the planet blocks – the bigger the planet, the more the star dims as it passes in front.
The trouble is, the transit method makes it much easier to find big planets that orbit close to their stars, and therefore don’t take long to get around them. To make a detection using the transit method, you have to see the light from a star dip not once but several times, to be sure it’s not just a fluke or some other artefact in the data. If you were looking at Earth from the other side of the galaxy, for example, you’d need to watch it for several years to see that dip happen more than once.
That’s why the majority of the planets astronomers have found so far are hot Jupiters – gas giants that orbit close to their star and have “years” that last just a few days, meaning within a week or two you could see such a planet transit several times.
And the smaller – and more Earth-like – the planet, the harder it becomes to see these dips in light.
Plato hopes to be able to work out the size of planets more accurately than ever before, and also to home in on Earth-sized rocky planets. One thing the team is doing is putting the telescope at what’s called the Lagrange 2 point, a million and a half miles away from Earth: much further than the moon but only a tiny fraction of the distance to our neighbouring planets. “It’s where you put experiments that need the most stable conditions,” says Pollacco. “The Earth doesn’t get in the way.”
The fact Plato is made up of several telescopes, compared with Kepler’s one, will also help correct for any errors in individual readings. And Plato has the advantage of a built-in support network of ground-based telescopes that can follow up on interesting observations and confirm its findings using other techniques.
Plato will also improve on past attempts by finding out as much as it can about the stars its planets orbit, which helps work out the mass of the planets themselves. This is in part using a technique Kepler wasn't really designed to use, called astroseismology – which is exactly what it sounds like. Just as vibrations that travel through Earth in the form of earthquakes can tell us something about the internal structure of our planet, vibrations that travel through stars can tell us about the star’s interior.
“We’ll also be able to characterise the internal structure of the star and the radius and the mass with great accuracy, which means all the measurements we make for the planet can be determined with great accuracy too,” says Pollacco.
By looking at the elements inside a star, astroseismology can also help work out its age, helping astronomers know if it'll be prone to spewing out the deadly flares of radiation typical of young stars, which could destroy life on an orbiting planet. It’ll also help astronomers to work out how old the planet is. “If you assume a planet is made at the same time the stars are made, you can then date the planet,” says Pollacco.
Despite these advances, Plato alone won’t be able to tell us for sure if somewhere is habitable. If it found our own solar system, for example, and it was able to see Venus, Earth, and Mars, they’d probably all look equally habitable, being roughly Earth-sized and in the habitable zone. But, from our perspective inside the solar system, it’s easy to see that’s not the case.
“If you didn't look at the atmospheres of Venus and Earth you wouldn't know how different these two planets are,” says Tinetti.
“Earth, Venus, and Mars are very similar in that they formed out of the same material, they're similar in composition, they're not that different in distance from their star,” Jonathan Tennyson, a professor of physics at University College London, tells BuzzFeed News. “But they are hugely different in terms of atmospheric composition and temperature and other characteristics. Mars is pretty much in the habitable zone, but it’s pretty certainly not inhabited at the moment. Venus has a runaway greenhouse effect.”
Tennyson, along with Tinetti, is involved in another British mission called Twinkle, which could help narrow down the hunt even further and should launch in 2020 – very soon in science terms.
“The idea was to go for a cheap, quick, three-year launch,” he tells BuzzFeed News. “I'm 62 – I'm not that young. All the other missions are 10 years away. For me that's not very attractive.”
Twinkle is a space telescope they hope will be powerful enough to analyse the atmospheres of Earth-like planets. It’s small, by space mission standards, and will be privately funded – an unusual model for a space experiment, most of which are funded by governments through bodies like the European Space Agency.
“The major challenge of Twinkle is not necessarily the science, it's to fund it,” says Tinetti. “Although it’s relatively cheap in terms of space satellites, we’re still talking about £50 million.” The fundraising is going well so far, she says, and the team have scientists queueing up to buy time on the telescope, which will sit in a polar orbit around Earth once it launches. They plan to start putting together a blueprint of the telescope this autumn, and to start building it in the spring. The current launch date is 2020.
Twinkle will look at exoplanets that have already been discovered in visible light and near-infrared wavelengths.
“The idea was to go for a cheap, quick, three-year launch"
Unlike most astronomers, Tennyson has spent his career looking at extremely small things. His expertise in quantum mechanics can further illuminate the search for Earth 2.0 because he works with a technique called spectroscopy to assess a planet’s atmosphere.
The idea is this: When a planet transits in front of a star, it blocks out light. With a planet like Earth, light will be blocked out by the rocky, solid bit of the planet, but the atmosphere will block light to some extent too. And because different light wavelengths correspond to different molecules, you can analyse the light to work out what molecules there are in the atmosphere of the planet.
“For instance, if you look at the starlight at a wavelength where water absorbs a lot of light, and there's a lot of water in the atmosphere of the planet, the planet will appear very big,” says Tennyson.
A decade ago, Tinetti and Tennyson, along with colleagues from France, Spain, and the US, used this technique to find water vapour in the atmosphere of an exoplanet. Their results were published in the journal Nature – only the second time water had been found on a planet outside of the solar system.
The planet it was discovered on is far from habitable, though. HD 189733b is 64 light years away and orbits a star in the constellation of Vulpecula. The gas giant is 30 times closer to its star than we are to the sun, with estimated temperatures over 900ºC on the side facing the sun – in short, a hot Jupiter.
“If, for instance, you look at water in a hot Jupiter exoplanet, that water is going to be at about a thousand degrees,” Tennyson says. “We know a lot about how water absorbs light in the Earth's atmosphere at room temperature, but it's very different at a thousand degrees.”
It’s much easier to do spectroscopy using the transit method on hot Jupiters because their atmospheres are much larger.
Using the method on an Earth-like planet is trickier. Earth is 12,000 km across, and its atmosphere is just 480 km thick. Using the transit method, the variations you’re going to see in the light, to give you clues about the atmosphere, are going to be extremely small. You need instruments that can pick that apart.
“You have to be really sensitive. At the moment we can't do that – it's much easier to look at Jupiter-like planets to start off with,” says Tennyson.
Once you know what the molecular fingerprint of a planet’s atmosphere is, and the planet’s temperature, you can start to work out what its climate might be like, and then you’ll begin to get an inkling of whether it’s really habitable or not. Tinetti explains: “If you have a lot of greenhouse gasses your planet will be much hotter. If you have many clouds and they're very reflective it could be cooler. There are a lot of factors that could change the climate on a planet and to find out about this you need to observe the atmosphere.”
The Twinkle team have a target list of molecules they want to look for, most of which have not been seen on exoplanets before. In terms of what they will see “it’s really an open book,” says Tennyson. “I can't tell you what we'll find because we're fishing in a pond that no one has fished in before.”
“The subject is only 20 years old; it's come on hugely fast. So I think this is just a stepping stone – we will get to the next stage very quickly,” says Tennyson. “Science is about taking steps, not giant leaps.”
These British projects could take significant steps towards an Earth 2.0 discovery in the near future, along with others like NASA’s James Webb Space Telescope, the successor to Hubble that’s launching in October next year.
In the next decade, Pollacco hopes Plato will have found some Earth-sized planets around sun-like stars that we might be able to look directly at using our powerful ground-based telescopes. By that point, we should also be able to investigate their atmospheres. “That's about 10 years away,” he says.
"We're fishing in a pond that no one has fished in before"
Tinetti cautions that it’s important to keep some perspective on our limitations when looking for habitable worlds outside the solar system. “Frankly it's not that we know this with a great level of precision. The only habitable world we know is the Earth,” she says.
“What we're seeing – even from the basic parameters we have – is that there's a huge diversity out there. Exoplanets seem to be much more diverse than the planets we have around the sun.”
And so to understand what a habitable planet looks like, she says, we really need a better grasp of all of the planets that exist outside the solar system. “The danger is, even if we find a few habitable planets, if we just look at those we might over-interpret or under-interpret what we see.”
The whole field of exoplanet science is constantly pushing the boundaries of what’s scientifically doable – which is what makes it so tricky to get right, but also so exciting. “The measurements you're trying to make are all right at the limit of what is possible, mostly beyond the limit in actual fact,” says Pollacco. “None of this is routine.”