DIFFERENT. Deadly. Alien. We've found 220 planets capable of sustaining life. Most are nothing like our own. Their stars are strange. Their weather is upside-down. Does this make us the alien freaks?
Astronomers have been busily scanning the skies for life-giving worlds since the 1990s. But it was not until the Kepler space telescope cast its eye over a narrow band of our galaxy that we got to see simply how many planets there really are out there.
Kepler alone has so far spotted 4000 of them. The fact that 0.05 per cent of these sit in orbits warm enough for liquid water has enormous implications for the existence of life.
But the real eye-opener is where most of these worlds are.
Most don't orbit yellow stars like our own.
Instead, we keep finding them around red dwarfs.
These are the most abundant stars in our galaxy, making up some 75 per cent of the Milky Way.
There's nothing particularly special about them.
They're dim. They're ruddy. They're small.
But everything from the colour of the sky, the behaviour of the winds and the shape of the seasons on the planets around them are unlike anything we have experienced. Their geography is also completely different to our own.
All this would drive life in unexpected directions.
It's an alternate reality that has set planetary scientists and exobiologists scrabbling to model and comprehend.
So far the odds appear loaded against life evolving around red dwarfs. But science is still at the stage of figuring out how many dice there are.
This isn't easy given how utterly alien these planets are.
But, given the sheer number of red dwarf stars in our galaxy, the chances are life will have gotten off the ground somewhere.
GIVER OF LIFE
The 'thing' about red dwarf stars like TRAPPIST-1 is their temperature.
They're not as hot as our Sun. But they live a lot longer.
"You have to be really very close in order for your planet to have temperatures on the surface like those we have on Earth," says associate professor in Space Science Graziella Caprarelli of the University of South Australia.
The closer you have a planet to a star, the more problems you may encounter. Solar flares, for example.
"Life is a game of numbers," Dr Caprarelli says.
"So you can be unlucky one day and you die, but your species may also be somewhere else so it survives. So events like solar flares in that context probably won't do much damage. Once life is established, it's established."
The biggest hurdle life faces under a red dwarf is its weak light.
Its radiation is also most intense at different wavelengths than we get from the Sun. Peak output - where it is the most intense, where you get the most photons - is in the infra-red range. From our Sun, the peak intensity is in the blue-green range.
So what would a world be like in the infra-red?
"If there is water on the planet, then water absorbs the infra-red radiation quite strongly," Dr Caprarelli says. "This water will be warmed up a lot. Much more than it is on Earth. So if the water is so much hotter, you can expect much more evaporation."
Which brings us to the heart of the matter: Clouds.
Once clouds have formed a planet can trap an atmosphere underneath. Their presence also helps keep life-giving water liquid.
"It is something that would work in favour of producing habitable planets around red dwarfs," she says.
EXPLORING A NEW WORLD
One particular characteristic will drive the shape of worlds orbiting stars like TRAPPIST-1: tidal lock.
These planets will have their spin gradually slowed over the course of a billion years. Eventually, they simply stop.
Essentially, the sun never rises or sets. It remains fixed at one point in the sky.
There is no day and night. Just one or the other - and many shades of red between.
It's something that has severe implications for climate and life. Exactly what remains uncertain. But science is attempting to extrapolate.
The dark side of a water-rich planet is almost certainly going to be one vast network of tightly packed glaciers and fields of ice.
The sunward side would be searing hot.
It's the transitional edge between the two which is most likely to be "just right".
It's a twilight zone.
It's called the "terminator", the boundary between night and day. Perpetual dusk. Eternal dawn.
It's also likely an environment of contrasts. In the shadows, ice. In the sunlight, dust. In between: life-giving liquid water.
Such water would create a potential 'ring of life'.
Conditions could range from steamy tropical swamps through to frigid ponds - all within a few hundred kilometres. Each sub-climate would be an almost permanent band encircling the entire planet. On Earth, of course, our seasons change as the planet wobbles in its spin.
But Dr Caprarelli says there is one major consequence of liquid water that could broaden a red dwarf world's habitability. Weather.
There's likely to be lots of it as hot air and evaporating water directly beneath the red sun rises - sucking cooler air in from below and depositing heat and moisture deep on the dark side.
"It comes back to the clouds," Dr Caprarelli says.
"Because it has just one side facing the sun, the planet will have temperature gradients. Because of these temperature gradients, it will generate very strong winds. And because of the winds, clouds would be redistributed around the planet in a more-or-less uniform way."
The outcome: More evenly balanced temperatures.
The fleshy body of this 'plant' is mostly underground, save for the spectacular rosette of petals that trap moisture and, with the aid of symbiotic microbes, harness sunlight.
While the mature organism is immobile and unthinking, its wind-blown seeds possess the limited degree of self-awareness necessary to navigate it to a site suitable for germination. - Dr Brian Choo
OF A DIFFERENT MOULD
Some speculate that red dwarf worlds would be dead, with all their water permanently locked away in ice. Whatever portion had faced their star would have boiled away into steam, only to fall as snow once it wafted over the dusk line.
Others believe there is much more to the geology of these worlds than that.
Dr Caprarelli suspects most red dwarf planets face a fate like Venus, with runaway greenhouse effects generated by excessive evaporation.
"But the beauty of red dwarfs is that there are so many of them," Dr Caprarelli says.
"Statistically, somewhere a red dwarf might have a planet that had all of the right conditions at the same time so that life can emerge and evolve."
A variety of landscapes would likely be carved out by such geological processes. These would then gradually erode under the influence of wind and rain.
Under a blanket of clouds, enormous glaciers near the rim of perpetual night and day could gradually be melted by the warm air circulating in from the sunward face. Some may even slip into the sunlight. Such glaciers would probably be sitting on sheets of liquid water - produced purely by the friction of their immense weight.
Vein-like valleys left from ancient geological activity could funnel this water deep into the sunward face, kept largely chilled by the permanent shadows cast by cliffs and eroded river banks.
Another mitigating factor for life would be that these planets will not have always been tidal locked, Dr Caprarelli says.
For their first billion years or so, they would have spun. This would give them internal friction, heating up their cores. Such geothermal activity would power a magnetosphere to protect the planet from flares.
Combined, these elements could produce an environment in which microorganisms could evolve.
With the right roll of the dice, these could then develop to harness the energy of their weak stars, she says. If they can do this by the time their planet grinds to a halt, life will have become well entrenched.
Such a planet would be primed for evolution.
Reporting: Jamie Seidel | Artwork: Steven Grice | Animation: Steven Grice |
Red dwarf climate: University of South Australia professor in Space Science, Dr GraziellaCaprarelli |
Astrobiology: Flinders University DECRA fellow, DR Brian Choo