Look up at the night sky and for each star you see, there’s the possibility we might not be alone.
If each of those stars had just one planet orbiting it, there could be countless worlds teeming with life.
There are hundreds of billions of stars in our galaxy alone but, the problem is, there isn’t yet an ‘answer’ to how many planets there are, never mind how many planets could support life.
For the vast majority of human history, the only planets we knew about were the ones in our neighbourhood: the seven other planets that orbit our Sun.
In just the last decade, our knowledge of planets around other stars – exoplanets – has boomed.
So what is the reality of discovering ‘extraterrestrial life’ and how close are we to figuring out our place in the galaxy and, eventually, the universe?
Exoplanets seem to be abundant in our galaxy and universe, while our knowledge of the chemistry of how life may have begun, abiogenesis, has come on leaps and bounds.
We have never been in a better position to discover more planets (and, if it exists, extraterrestrial life) and we’re only at the tip of a universe-sized iceberg.
If human history was condensed to 24 hours, these discoveries would have happened in the last second. It was only in 1995 when astronomers found that our solar system isn’t unique.
A star called Pegasi 51, similar in size to the Sun, had a wobble in its rotation that could only be explained by the existence of a planet-sized object.
Pegasi 51 b, made of gas like our own Jupiter but closer to its star than ultra hot Mercury, was the first example of an exoplanet, a star-orbiting object outside our solar system.
Swiss astronomer Prof Didier Queloz, who won this year’s Nobel Prize in physics for the discovery, has since found hundreds more using the same technique.
‘Science is about gambling, vision, and trying to push the boundaries,’ Prof Queloz tells Metro.co.uk.
‘You don’t push the boundaries if you don’t have vision. You don’t fly to the moon if you try to improve the steam engine.’
At the time, looking for exoplanets was not a focus for astronomers but the field has expanded rapidly since.
Since Pegasi 51 b was found 25 years ago, 4,099 confirmed exoplanets have been found.
But with billions of stars in our own Milky Way, and so potentially billions of planets, so much remains unknown:
How many are there? What defines a planet? Could any of them harbour life?
And even before you can look for planets, you need to know what you’re looking for.
‘There is no definition for exoplanets,’ says Prof Queloz.
‘There is only a definition for the planet, and this definition of the planet is related to the solar system.’
In 2005, members of the International Astronomical Union (IAU) met to discuss updating the definition of a planet inside the solar system.
When Pluto was first discovered, it was assumed to be the only object of its size inside the Kuiper belt, an area of rocks and dust just farther from the Sun than Neptune.
‘Under the old definition, we would end up with 100,000 planets in the solar system, which is kind of crazy,’ Dr Matthew Bothwell, outreach astronomer at the University of Cambridge, tells Metro.co.uk
‘It was a bit of a fudge to make it so we’d end up with a reasonable number of planets.’
The definition the IAU settled on – spherical, orbiting the sun and being the only big thing in its orbit – famously demoted Pluto from its planet status, yet the definition remained.
But the IAU never settled on a galaxy-wide definition of a planet.
This definition has become a topic of hot debate in the astronomy community: should it be based on the way the planet’s formed, on its structure or how big it is?
One area that holds some hope is the European Space Agency’s mission Gaia, a nine-year survey of the skies now in its fifth year.
‘There is a hint that we may have a kind of a [size] boundary around the 20-30 Jupiter mass mark,’ says Prof Queloz.
In space, there is almost every size of object, from rocky asteroids to gargantuan stars.
But starting at a level around 30 times of Jupiter’s mass, there is a strange absence of planets.
‘The more you move away from a Jupiter mass the less you find objects, then there is a sort of a gap of objects and it starts rising again.’
It’s all so new that clear definitions are tricky to pin down: objects called brown dwarfs are bigger than planets (starting around 15 times the size of Jupiter) but not quite stars, even though they are formed in similar ways.
There is no agreed consensus about exactly where planets end and brown dwarfs begin, or even where brown dwarfs end and stars begin.
It comes back to the fact that a planet does not have a clear definition.
Anyway, once you’ve decided what a planet should be, the next problem comes with how to go about finding them.
Prof Didier Queloz’s wobble-technique, known as Doppler spectroscopy, was used to find the first and subsequently hundreds more planet, but astronomers have devised more methods since.
Transit photometry, where a star can be seen to briefly dim as a planet pass across it, has found the bulk of the 4,000-odd exoplanets.
Scientists are most excited by what’s called ‘direct imaging’.
‘The Holy Grail is really getting pictures of a planet,’ says Prof Queloz.
‘The reason we need pictures is because that’s the only way to analyse in great detail what is going on in the atmosphere of these planets.’
But there’s a problem:
‘Stars are really big and bright, planets are small and faint,’ says Dr Bothwell.
‘You have to basically block out the light from the star, which is a really difficult thing to do.’
An extravagant solution called a starshade, a large disc which would act as a light block, has been proposed to accompany NASA’s soon-to-launch James Webb Telescope.
‘The screen will position itself exactly right to block the light from the star, like using your finger to block out a distant streetlight,’ says Dr Bothwell.
‘Then, hopefully, we’ll be able to see the planets orbiting underneath.’
Alongside the powerful telescopes like James Webb, new techniques are being developed to make more of the galaxy visible.
The techniques of interferometry allow the illusion of a telescope 10 miles wide to be created.
Rather than building one that size, which would be practically impossible, you can build two telescopes 10 miles apart and they will function the same.
These sorts of developments have made many scientists enthusiastic about the near-future.
‘We can call it the golden age of astrophysics,’ says Prof Queloz.
‘If you look at the last five years, there have been two Nobel Prizes dealing just with astrophysics results.’
We’re better placed now than at any point in history to find the planets further than anyone has ever seen.
But the end goal for many astronomers isn’t to find all the planets in existence.
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‘The emphasis is trying to identify the planets that are orbiting stars around us; they are easiest to observe and to measure,’ says Prof Didier Queloz.
‘I would rather spend all the money right now to detect planets more like the solar system and trying to search for life, than trying to find planets in other galaxies.’
This eagerness is shared by many astrophysicists, but not all agree on how likely it is.
‘We just don’t know what the odds of life are,’ says Dr Bothwell.
‘If the odds of life emerging on a planet are 10%, then obviously we live in a galaxy full of life.
‘But if the odds of life emerging are a trillion to one, which they could be, then it would stand to reason that we are the only ones in the galaxy.’
Despite the fact that astronomers are literally and metaphorically searching in the dark, they have received considerable support from both public and private bodies.
The US-government backed NASA conducted a vast survey of 530,506 stars during The Kepler Mission and Google supplied hardware firepower for a machine learning-based analysis.
Russian billionaire Yuri Milner has launched his own $100 million (£78m) search for life, called Breakthrough Listen.
However, it’s unlikely that big business will ever get too deeply invested.
‘It’s not going to be a huge thing for companies because astronomy is not profit-driven,’ says Dr Bothwell.
‘You’re not going to make a fortune from astronomy, it’s something that we do for the benefit of the human species.’
While arguments over how much responsibility private and public companies should share, there’s also a third actor.
‘In recent years there has been an explosion in what we call citizen science,’ says Dr Bothwell.
‘There have been citizen science projects where people will search through Kepler data to find planets in a way that big organisations can’t.
‘Science is becoming more and more democratised as time goes on, which is always a really good thing.’
Could we ever find all the planets?
‘The field is only a couple of decades old,’ says Dr Bothwell.
‘We’ve already gone from the point where we’re discovering a handful of these things, to finding thousands of them.’
‘I see no reason why it shouldn’t just continue to exponentially increase.’
Given there are at least 200 billion stars in the Milky Way alone, some experts have put the number of planets in our galaxy at 100 billion.
Combine that with the 100 billion galaxies predicted by the Hubble space telescope, and the total number of possible planets becomes mind-boggling, if it wasn’t already.
But experts have urged caution at mapping them any sort of significant proportion of them for the time being.
It’s not impossible but for many astronomers, mapping the galaxy, let alone the entire universe, might be both infeasible and unnecessary.
‘It’s going to take a while. Let’s go to Mars first,’ says Prof Queloz.
‘Let’s keep this question for my grandchildren.’
The Future Of Everything
This piece is part of Metro.co.uk’s series The Future Of Everything .
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