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Monday 30 July 2018

Answers for Authors: What would life be like on a planet orbiting a red dwarf star? (re-posted)

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A tiny star 40 light years from Earth, called TRAPPIST-1*, made some major waves a while back: It has seven planets, all roughly the same size as Earth, all with some chance of habitable conditions.

Above: An artists impression of the TRAPPIST-1 solar system.

So the question has been asked: If a civilisation were located on one of the TRAPPIST-1 planets, what would be different to our civilisation?

Short answer? A lot. 

Long answer?
First lets look at the TRAPPIST-1 star itself. It's an ultra cool red dwarf, the smallest kind of true star** you can get - just 8% the Sun's mass, and barely wider than our planet Jupiter. That gives it a very different look to our Sun: In a nearby planet's sky it would appear orange coloured, and dimmer. Paradoxically, if you're viewing from a habitable world, it would also look slightly bigger as a red dwaf's habitable zone is much closer in than  our Sun's. Red dwarfs also suffer from huge star spots, giving it a mottled look - and these come as part of a package stellar violence: TRAPPIST-1 spits out powerful particle storms, and a lot of UV and X-ray radiation.  Although invisible except to specialised instruments, coronal mass ejection particle storms (responsible for auroras on Earth) damage the electronics of satellites and spacecraft, cause cancers in astronauts, and cause radio blackouts and damaging power surges here on Earth. Those from TRAPPIST-1 would hit its planets like the strongest storms ever spat out by our Sun.

But being dim, orange, angry, and ugly is no obstacle to success - look at Donald Trump. Being so small means TRAPPIST 1 burns its nuclear fuel much more slowly and efficiently than our sun, so it will live thousands of times longer.  

I truly hope the Trump doesn't live thousands of times longer than expected . Though he is full of surprises... horribly full of them...

So that's TRAPPIST-1 itself: Small, orange, angry, and tenacious. What about the planets around it? 

There are seven of them and, because such a small star only has small gravity, they must orbit much closer than any of our planets to stay bound.  The furthest of them still has an orbit much smaller than Mercury's. But the cool stars small size means these worlds don't scorch - that outermost world is probably an ice ball! The habitable zone, where planets are most likely to have liquid water, is closer still - three of the planets are in it. 

So, making the assumption that any civilisation will be based on one of the habitable zone planets, we can come to a few conclusions: 

  • The Sun will look orange, bigger in the sky than ours, and dimmer to look at. There aurora will be spectacular - assuming the planet has a strong protective magnetic field. 
  • If the planet doesn't... the very powerful, close range, solar storms will have damaged and thinned the atmosphere, leaving it more like a big version if Mars than Earth.  
  • The year will be a couple of weeks long at the most - if the planet has seasons each would only last a couple of days. 
  • These planet's close range to their sun might make them 'tidally locked' - so one side  always faces the sun, and one side always faces away. That would make the sun stand still in the sky, bathing half the planet in eternal day, and the other half in eternal night. A thick atmosphere  will balance the extreme temperature  differences out a bit, but one side would still be a place of bitter cold and ice caps,the other a realm of deserts.  Only a strip of the planet along the day/night boundary would be temperate,  limiting the civilisation's ability to spread. 
  • Because of this solar system's  small size the other planets would look much bigger in the sky than the planets of our solar system - often bigger than the moon from Earth. So if more than one planet had a civilisation each could see the glow of each other's city lights at night. 
  • Plantlife would be a different colour from Earth's: Our plants are green, because that's the brightest colour in the Sun's spectrum (see 'what colour is the Sun'), and the plants need to reflect the brightest light to avoid cell damage. But a red dwarf's light is fainter, so plants on a TRAPPIST 1 planet might well be black, to absorb as much as possible .
  •  The sky would also be a different: Our sky is blue thanks to scattered blue light from the Sun, which TRAPPIST 1 produces less of - so it's sky would be much darker. Since the blue wavelengths  are absent the sky might even be green, as that colour is the more abundant wavelength short enough to scatter off air molecules.
What about travel between those planets? The orbits of these worlds are the same sort of distance apart as the Earth and Moon (which are only three days travel apart using 1960's rocket technology).  Because the orbits are so short launch windows would occur on a weekly basis... Which makes the TRAPPIST 1 solar system sounds like an ideal place for a spacefaring civilisation.

It's not that we're complaining about how hard space travel is universe... well... actually it is.

But this teeny star system also has hazards - the planets will need heavy duty magnetic fields to protect them from their ill tempered sun,  and some radiation will get through anyway.  A surface civilisation will need to be very X-ray tolerant. On top of that, the closeness of the planets means they tug on each other gravitationally, changing each other's orbits over millennia and causing huge climate shifts.

So, to sum up? A civilisation around TRAPPIST 1 would be able to travel between the worlds there in days, and their seasons would race past in mere days. The plants would be orange (assuming they worked the same way as ours), and the sun would be distinctly orange in the dark sky, and would look larger than ours, and the other worlds would loom in the sky, as big as the Moon looks from Earth. But they will need to be hardy: Only thin strips of their worlds, along the terminators, would be habitable - the rest of their surfaces will be gripped in eternal roasting day or endless freezing night. Radiation levels will be high, with frequent  solar storms powerful enough to cause worldwide radio blackouts, power grid surges, a severely damage satellites. Humans would face elevated cancer risk, even on the ground, and in space a storm could be lethal.

A fascinating setting - but not for the fainthearted...

*Named after the beer brewed by Trappist monks . NAMED AFTER BEER! 
*A true star fuses hydrogen for fuel. Objects much smaller than TRAPPIST-1 can only fuse deuterium, and are called 'Brown dwarfs'.

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