Mysteries they are, distant planets bobbing through the deep darkness of the cosmos, entire worlds caught in the gravitational pull of a star other than the sun. Astronomers also call them exoplanets, and thanks to modern space telescopes such as Tess and predecessor Kepler we know of more than five thousand.
But what those distant worlds look like—whether peaceful streams ripple across their surfaces or violent tornadoes rage, whether volcanoes erupt or fresh showers fall—no one has yet seen with their own eyes.
In the solar system it is very different. ‘Our’ eight planets have been extensively photographed: Saturn with its enchanting rings, Mars with its ocher desert plains and the icy blue of Uranus and Neptune. We’ve known the sight even from distant dwarf planet Pluto since 2015, with that distinctive heart pattern on its surface.
Yet those eight known worlds pale in comparison to the total number that can be found in the Milky Way, the galaxy in which our solar system revolves. It contains about a hundred billion planets in total, astronomers estimate.
Of the exoplanets that we already know, we usually know that they are exotic places, with equally exotic names. ‘PSR B1257+12A’, for example, a world where the radiation is so powerful that a person would instantly turn into a pile of dust. Or ‘HD 189733B’, where it rains – no, really – molten glass and blows with the combined wind force of seventy hurricanes.
At least that’s what astronomers think. Because such places are so dizzyingly far away that you can’t take a look. For example, HD 189733B is roughly 63 light-years away, some 596,438,640,000,000 kilometers, so far that even our best spacecraft can practically never fly there.
We only know the existence of such places because they reveal themselves indirectly, for example when they cross in front of their star and the light dims for a while. But take a picture? No. Such a planet in the sky is simply too small for that.
This means that we can only determine all those presumed properties – from glass showers to deadly radiation – indirectly. By how much light a planet blocks (a measure of its size), for example, how hard it makes the star it orbits wobble (that gives its weight) or by its distance from that star (a measure of its temperature) to be accurately determined.
‘In my research I am mainly looking for places where life could be possible,’ says exoplanet researcher Ignas Snellen of Leiden University. ‘With conventional telescopes you always come up against a limit. You might be able to see what kind of substances are in the atmosphere, recognize whether oxygen is present anywhere on the planet, take a stab at the local weather,’ he says.
More seems impossible in the short term. Because if you really want to photograph a distant exoplanet, including visible details on the surface, you will soon have to build a giant telescope that is roughly a thousand kilometers in size, says Snellen. In comparison, the mirror of the Hubble Space Telescope is 2.4 meters. That of the brand new space telescope James Webb 6.5 meters. So, to say the least, you still have some steps to go.
But what if, with some ruse, it still works? ‘Then you can suddenly go much further in exoplanet research,’ he says. ‘Then you can not only see whether a planet is suitable for life, but also discover what kind of life that is. You can search for water or continents. To green vegetation. Ha, maybe you even see cities or something like that, and you immediately know whether intelligent life exists on such a planet.’
Here are three speculative ideas for achieving that.
A telescope that uses the sun’s gravity to image exoplanets. That’s the crazy-sounding idea that astrophysicists discussed in earnest in early May in the trade magazine The Astrophysical Journal†
The underlying principle stems from Albert Einstein’s general theory of relativity, which states that masses can bend a light beam, similar to how light changes direction in a lens. Perhaps then you could also use the sun, with its large mass, as a gigantic telescope lens, the researchers thought.
On paper, this produces images that are just as sharp as if you were using a telescope that is twenty times the size of the entire Earth, they calculate in the article, a rough 130 times larger than the minimum size you would expect. Snellen would need for exoplanet photography.
“With this technology, we want to take a picture of a planet 100 light-years away, with the same impact as the first picture taken by the moon mission Apollo 8 of Earth,” said lead researcher Bruce Macintosh. in the university press release about the article.
All it takes? A space telescope roughly the size of the Hubble. But, and that’s the crux: it must be six hundred times further from the sun than the earth. Even in the most optimistic scenario, you won’t be able to place a telescope there for about forty years.
And then practice is probably even more difficult, says astrophysicist Matthew Kenworthy of Leiden University. ‘The image you create with such a gravitational lens is completely mixed up. For the reconstruction you then have to know the gravity of the sun very accurately’, he says. “More accurate than we know it now, I suppose.”
In addition, you should really do a test mission first and hope that it doesn’t run into problems that make the entire enterprise impossible after all. “I’d say something like this won’t be possible for a few hundred years at the earliest.”
How about something different? Perhaps the idea of telescope maker Chris Walker, affiliated with the University of Arizona, offers a solution. He thinks that in the short term – in a few years, if someone gives him the budget – he will be able to launch a space telescope with a diameter of 35 meters.
His proposal, he previously told de Volkskrant† a monster that inflates itself in space like a balloon. With a transparent top and a mirrored bottom on the inside of the sphere, creating a large spherical mirror that collects the light and sends it to a detector.
Impossible? No, Walker thinks. In the 1960s, Nasa already sent inflatable spheres into space with its Echo missions with a diameter of about forty.
“Inflatable telescopes are a great idea,” says Kenworthy. “Much more plausible than some of the other plans, anyway.”
But: to get a sharp picture of distant planets, a few tens of meters is not enough. That is why Walker and colleagues also fantasize about the ultimate sequel, where you can launch really gigantic telescopes, variants that could eventually become large enough to photograph exoplanets. ‘I think that on those planets we can look at land masses, see clouds pass by and oceans slosh. Real science fiction stuff,” Walker said. But: that also takes development time. Earlier than the year 2100-something we should not expect anything like that.
“We can always dig out the far side of the moon and hang a giant telescope in it,” says Kenworthy, when asked to really think outside the box. Make the pit and the accompanying telescope big enough and you can start looking at distant planets.
Ho, stop, wait: maybe that can also be done with a little more feeling for cosmic heritage. By not blowing off a piece, but simply placing a network of radio telescopes there. At least that is the plan that astronomer Marc Klein Wolt (Radboud University) is now carrying out for the European space agency Esa. Together with his colleagues, he hopes to place a network of a thousand radio antennas on the far side of the moon, of which you can link the measurement data together afterwards to effectively create a giant telescope. The first antenna is expected to go to the moon in 2028.
And while such a telescope promises to be a powerful instrument – with which you can peer very deep into the cosmos – you can not yet take a picture of the surface of a distant exoplanet with it. ‘But something like this is possible with our technology’, thinks Klein Wolt. You just need a lot more antennas for it than planned. ‘You have to think of ten thousand elements or something, spread over a very large surface.’
And it may be a bit predictable, but that plan also takes time. Sometime during the 1930s, the first thousand antennas should be on the moon. There are no plans for more, but it is to be expected that it will take you a while to place them, if someone would give the green light.
Anyone hoping to see the water splashing on a planet outside our solar system with their own eyes should therefore keep their fingers crossed for an unexpected technological breakthrough, or bet on breaking the human age record (122 years and 164 days). In all other cases, that photo will be a first for our children or grandchildren. It’s nice: can we make them a bit nice for that now?