It’s one of the biggest unanswered questions in science – if there’s life beyond Earth, where is it?
While you might think the answer is ‘far far away’, surprisingly, it could be much closer to home than we thought.
Our very own solar system is now home to almost 300 moons – some of which could have the perfect conditions for life.
But which ones are the most promising candidates?
From Jupiter’s moon, Europa, to Saturn’s satellite, Enceladus, this graphic reveals the moons most likely to harbor aliens.
Moons – also referred to as ‘natural satellites’ – are defined as such when they orbit a planet. There’s a little under 300 moons that we know about in our solar system, but the best candidates for life include Europa, Enceladus and Callisto
Scientists have just discovered three new moons around Uranus and Neptune. Pictured is the Uranian moon, provisionally called S/2023 U1. Uranus is just off the field of view in the upper left, as seen by the increased scattered light
According to experts, the planets most likely to have some for of life include Saturn’s moon Enceladus, which shoots out plumes of water from a hidden liquid ocean.
Jupiter’s moon Europa has CO2 on its surface (on Earth a by-product of cell function), while the larger Jupiter moon Callisto may have a salty ocean beneath its surface – a potential habitat for life.
Jupiter’s Ganymede – the solar system’s largest moon – is thought to have water and rock interacting, key for the development of life.
And Neptune’s moon Triton is known to be geologically active and has an atmosphere made mostly of nitrogen, just like Earth.
Scientists recently revealed that they’d found two more moons – one around Uranus and two around Neptune.
Amazingly, there are still ‘thousands’ more moons likely awaiting discovery in our solar system, according to NASA.
Dr Robert Massey, deputy executive director of the Royal Astronomical Society, agrees with NASA that there are many more moons that we don’t know about.
But they are all likely way smaller than the above candidates – a few hundred or even tens of miles in diameter.
‘Undiscovered moons are likely around the giant planets – Jupiter, Saturn, Uranus and Neptune,’ Dr Massey told MailOnline.
‘But we can be confident that we’ve discovered all the larger moons in the solar system around other planets.
These images show the two newly-found moons around Neptune – provisionally called S/2002 (left) and S/2021 N1 (right). S/2002 N5 (left) is somewhat bright and so is clear in the image, but with S/2021 N1, the moon is very faint. It can be just about seen as a black point of light right in the middle of the image. This one is very hard to see since it is so faint, the faintest moon ever found around any planet
Pictured, Jupiter’s moon Europa, where an ocean hidden under kilometers of ice is considered a prime candidate for extraterrestrial life. Scientists have found carbon dioxide (CO2) on Europa’s surface and the next step is determining where it came from
‘Moons discovered now are typically much smaller – no more than a few kilometres across – and therefore much fainter, so finding them needs larger and more sensitive telescopes.’
Last week, scientists found three new moons – two orbiting Neptune, and one orbiting Uranus.
However, Dr Massey thinks it’s ‘extremely unlikely’ that any of the three newly-discovered moons are candidates for alien life, because they’re too small to have an atmosphere and aren’t thought to have subsurface oceans.
Generally, the smaller the moon the less likely it is to harbor life, because to have life it would need to have an atmosphere.
Martin Hendry, a professor at the University of Glasgow’s school of physics and astronomy, told MailOnline: ‘If a moon is too small, its gravity isn’t strong enough to hold onto an atmosphere so there’s nothing for life to breathe.
‘If the moon is really small – e.g. small enough that we haven’t detected it yet – then the tidal forces will almost certainly be strong enough to tear the moon apart.’
Even with the most powerful ground-based telescopes, many of the moons that we don’t know about are too faint to see.
‘Planets and moons don’t shine by themselves – they just reflect light from the sun,’ said Professor Hendry.
‘For a moon orbiting quite far from a planet which itself is very far away from the sun, there’s not a lot of sunlight to reflect.
‘That’s why it takes such powerful telescopes to see them at all, and why they weren’t discovered before now.’
Dr Scott S. Sheppard, an astronomer at the Carnegie Institution for Science who found the new moons around Uranus and Neptune, doesn’t think there are as many as ‘thousands’ of moons still to be found as NASA suggests.
Enceladus – Saturn’s sixth-largest moon – is a frozen sphere just 313 miles in diameter (about one-seventh the diameter of Earth’s moon). It is pictured in this image captured by NASA’s Cassini spacecraft
Neptune’s moon Triton (pictured) is known to be geologically active and has an atmosphere made mostly of nitrogen, just like Earth
But he agreed that we currently do not have the ability to discover them because they are ‘too small and faint for current technology to see’.
‘Our current largest telescopes in the world are still limited in how deep they can efficiently image, so that is why we have those size limits for moon discovery,’ he told MailOnline.
‘All of these unfound moons are small, being only a few to tens of kilometres in size, so would not be ideal for current life
‘But they could have the ingredients for life on their surfaces, as they are thought to be ice rich with organics on their surfaces.
‘These moons are the leftover remnants of a once larger population of objects that were incorporated into the formation of the planets, so studying these moons gives us a better understanding of what went into the making of the planets.’
His three new discoveries bring Neptune‘s total known moons to 16, while Uranus now has 28, although this is still modest compared to the solar system’s two biggest planets.
Jupiter has 95 moons and Saturn has a whopping 146 – and the figure is rising regularly.
Of course, the probability of life existing outside of Earth gets greater when you take into account planets and moons outside our solar system – known as ‘exoplanets’ and ‘exomoons’.
NASA thinks there are billions of exoplanets alone, a certain proportion of which surely have similar conditions to Earth for life to thrive.