The arrangement of planets we see in the solar system is just one of many, many possibilities. The grand expanse of the universe holds a tremendous variety of planetary systems and we need not go far to find something unusual.
Trappist-1 is a small dwarf star, not much bigger than the planet Jupiter, located about forty light years away in the constellation Aquarius – a small distance in comparison to the 100,000 light-year breadth of the Milky Way. Revolving around this small star is something remarkable: seven rocky planets, all of them close to the size of Earth, all of them potentially the right temperature for liquid water.
They’re all warm because they’re huddled close together around their small star. The seventh planet, the farthest one out, is still six times closer to Trappist-1 than Mercury is to the Sun.
If you were on the surface any of Trappist-1’s seven planets, the sky would be filled with the other six, some of them close enough and large enough that they’d appear several times bigger than the moon does in our sky.
Three are located within what is traditionally considered the habitable zone, but all of them could have liquid water because of how near they are to each other. It could also be producing some strange waves in that water. The tides rise and fall on Earth with the motion of the Moon, but seven large planets all close together would produce even bigger tidal effects in each other.
It wouldn’t be random, though. All seven are in orbital resonance, meaning that the amount of time each takes to make an orbit, relative to its neighbors, forms almost perfect whole numbers. For every twenty-four orbits the closest planet makes, the next one out takes fifteen, and the next one out takes nine, and the next one six, and then four, and then three, and then two.
There is nowhere else in the universe that we know about (yet) that forms this long of a chain of planets in orbital resonance. All seven planets dance in harmony with each other and they will be doing that dance for a very, very long time.
The Sun has been around for about five billion years and should be around for another five or so billion years. But ultra cool dwarf stars like Trappist-1 have a lifespan estimated to exceed a hundred billion years. The universe has been around only a small fraction of that time – about 13.7 billion years. All cool dwarf stars in the universe are still young and have billions and billions of years to go. A huge amount of time means a huge amount of possibilities.
One possibility is panspermia. Tests in laboratories and in low Earth orbit suggest that some simple, single-cell organisms can survive ejection into space by asteroid impacts and atmospheric reentry.
If life formed on one planet in the Trappist-1 system, it could have spread to the other planets relatively quickly. The chances of panspermia occurring increases by leaps and bounds with seven planets all close to each other.
That’s just one possibility. There many others. All life needs is time and Trappist-1 will have plenty of time.
Temperate Earth-sized planets transiting a nearby ultracool dwarf star – European Southern Observatory
Potentially Biogenic Carbon Preserved in a 4.1-Billion-Year-Old Zircon – National Academy of Sciences
Enhanced interplanetary panspermia in the TRAPPIST-1 system – Cornell University Library
Panspermia: A Promising Field of Research – Astrobiology Science Conference 2010
Illustration of Trappist-1 by NASA/JPL-Caltech.