6 Things You Should Know About the New Planet Discovery

Last week NASA made the historic announcement that seven Earth-sized planets had been found around a single star. What do you need to know about the new TRAPPIST-1 system?

Sabrina Stierwalt, PhD,
February 28, 2017
Episode #228

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Last week NASA made the historic announcement that seven Earth-sized planets  had been found around a single star. Astronomers have found Earth-sized planets before, but never so many orbiting the same star, in this case an ultracool M-dwarf known as TRAPPIST-1.

Amidst the flurry of press coverage this announcement has inspired, here are six important points summarizing both why the star TRAPPIST-1 and its seven planets are so important, as well as how this discovery raises even more questions about life on other planets.

1. Three of the planets are crammed into the star’s habitable zone.

Some stars burn brighter (and hotter) than others, and so the range in distances from a star that result in temperatures amenable to liquid water will vary from star to star. Since water is a key ingredient for life like us, this range of radii where temperatures are not too cold and not too hot but instead “just right” are sometimes called the “Goldilocks region” or the star’s habitable zone.

Of the seven Earth-sized planets found around TRAPPIST-1, three reside in the habitable zone. However, distance from its host star isn’t the only determining factor in a planet’s temperature: atmosphere also plays a big role. For example, here in our solar system, Venus, Earth, and Mars are all in our sun’s habitable zone. Unfortunately for our potential future travel plans, Venus is experiencing a runaway greenhouse effect that creates a very thick atmosphere which is very good at trapping in heat like an extremely warm blanket. On the other hand, the atmosphere on Mars is quite thin, leading to large temperature changes from day to night. Thus, give or take a little atmosphere, any one of the seven TRAPPIST-1 planets could host temperatures hospitable to life, but the ones in the habitable zone have a better chance.

2. TRAPPIST-1 is not a star like our sun.

Our sun is an average G-type star, which means it emits the most light at white to yellow wavelengths and has a surface temperature of 5,000-6,000 Kelvin, which is somewhere in the middle of the range of temperatures stars are known to exhibit. TRAPPIST-1 is an ultracool M-dwarf which means it is on the order of 10 times smaller than our Sun – in fact, it’s barely bigger than Jupiter – and has a surface temperature less than half that of the Sun’s.

This cooler temperature means that the habitable zone for TRAPPIST-1 is much closer to the star than is the case in our own solar system.  The three planets found in habitable zone for TRAPPIST-1 have orbital periods between 6 to 12 days, meaning that “a year” on one of these newly discovered planets spans less than what we would call 2 weeks here on Earth. In fact, the whole system is barely larger than Mercury’s orbit around the Sun. Such close quarters make it even more impressive that 3 planets are found crammed into the habitable zone.

Another important distinction between stars like the Sun and stars like TRAPPIST-1, is their activity. According to Dr. Bárbara Rojas Ayala, an expert on M-dwarfs and an assistant professor in Santiago, Chile, ultracool M-dwarfs can be very active. “Their flares can be quite powerful and thus the high energy radiation they produce in the form of X-rays and ultraviolet radiation can affect the atmospheres – and thus the habitability – of their planets.”


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