TRAPPIST-1: The Most Habitable Exoplanet
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TRAPPIST-1: The Most Habitable Exoplanet

By Noe Matthew

For decades, scientists have been discussing what other planets might be habitable to humans. Because of recent concerns over climate change, the search for a new, healthy planet has drastically increased in relevance and magnitude. So far, scientists have found around 59 habitable exoplanets. But which one is the most habitable? The answer to this question may lie in TRAPPIST-1.

Discovered in February 2017, TRAPPIST-1 is a star system 40 light years away from Earth (around 240 trillion miles or 386 trillion kilometers). It has existed for 7.6 billion years, around three billion years longer than our solar system. The system centers around a cool red dwarf star. The surface temperature of this star is around 2,500 Kelvin, less than half the temperature of our sun. For a star system like ours, the red dwarf wouldn’t have enough heat for the planets to be habitable. However, TRAPPIST-1’s planets are close enough to the star that they still have similar temperatures to the planets in our solar system.

What truly distinguishes TRAPPIST-1 is that the star system contains not one, but seven Earth-like planets: TRAPPIST-1b, -1c, -1d, -1e, -1f, -1g, and -1h. So which one is the most habitable? At the center of the system is TRAPPIST-1b and 1c, the planets closest to the red dwarf star. Both planets exhibit similar mass, terrain, and atmosphere to Earth, but their close proximity to the star causes the planets to be very hot, making them uninhabitable. What about the planets on the outer edge of the star system, then? Well, TRAPPIST-1f, -1g, and -1h are also similar to Earth, but the cold temperatures make them uninhabitable. We’re now left with the planets in the middle: TRAPPIST-1d and TRAPPIST-1e. These planets exist in “the habitable zone.” They have lots of liquid water on them, possibly more than the Earth’s oceans! But which one is more habitable? Well, TRAPPIST-1d’s atmosphere may be too thin. So we have our winner: TRAPPIST-1e!

There’s one catch, though. We have to consider the tidal lock of  TRAPPIST-1e. A tidal lock is when an object orbiting a bigger object gets trapped in that bigger object’s gravitational pull, causing the smaller object only to show one face to the bigger object. If this is the case for TRAPPIST-1e and the red dwarf star, the side of the planet facing the star would be much hotter than the part that is not, and the other side would be much colder. Depending on how drastic this effect is, there may only be a small sliver of habitable area on this planet. This sliver is called the terminator line, an area where the cold and hot zones of the planet meet. This would be the place most suitable for life on the planet. So, the most habitable place in the universe, other than Earth, may be a tiny sliver of a small planet in the TRAPPIST-1 system!