March 24, 2017
In February 2016, the TRAPPIST telescope in Chile discovered that a red dwarf star nearly 40 lightyears from Earth holds seven exoplanets in its orbit. Of course, the discovery of exoplanets is nothing new, but this star system is a unique find. The seven planets surrounding the star, TRAPPIST-1, are all approximately the size of Earth. Finding Earth-like planets is always exciting news, but we have never before found seven of them orbiting the same star. Some of these planets may even contain the necessary conditions to support biological life.
Due to the immense distance between us and TRAPPIST-1, it is astounding astronomers have been able to detect the planets and discern any information at all.
How Astronomers Detect Exoplanets
Finding exoplanets is a trickier process than you might imagine. However dim, we can observe any star that emits enough light to reach Earth, but a planet itself emits no light. This makes locating these planets far more difficult, because many cannot be seen with our telescopes. In these instances, astronomers rely on indirect methods, such as the radial velocity method and transit method.
These two methods operate on a common principle. Rather than looking for planets directly, they calculate the kind of influence a planet would have on a stars behavior and search for stars that match those conditions. The transit method provides the perfect example.
The transit method can detect the presence of exoplanets by monitoring dips in the star’s light emissions. Imagine a star in space with a planet orbiting around it. As that planet passes between the star and the observer’s telescope, the planet will obstruct a portion of the star’s light. On Earth, we’ll be able to detect this dip in light emissions, and if the dip occurs in consistent time intervals, it is evidence of an exoplanet. Of course, this isn’t a perfect method and has a history of yielding false positives. Therefore, astronomers are sure to confirm any suspected planets through other means.
The radial velocity method works quite differently but can also be used to detect exoplanets. Using the radial velocity method, astronomers search for shifts in the color of light emitted by a star. If a star is orbited by a planet with enough mass, the orbit will actually shift the location of the star in space. As it revolves around the star, the planet pulls the star out of place slightly, and because the planet is always in motion, it causes the star to move in small circles rather than remaining stationary. As the star moves closer and farther away from Earth on its small rotational axis, this variance in distance actually affects the color of the light emissions on the electromagnetic spectrum. If astronomers detect this variance, they have a lead on a potential exoplanet. The radial velocity method has proven exceptionally useful and lead to the discovery of 582 exoplanets.
TRAPPIST-1 and Its Seven Exoplanets
There is a tremendous amount of information we don’t know about these planets, but the most compelling aspect of the TRAPPIST-1 discovery (so far) is how similar these planets are to Earth. Each one is similar in size to Earth or Venus. Compare that to even our own solar system where planets will vary radically in size.
Building upon that information, three of the seven planets even fall within the habitable zone or “goldilocks zone.” The habitable zone refers to the range of distances a planet can be from a star to create a climate moderate enough to support liquid water and the kind of life found on Earth. We have not confirmed whether some or all of these planets contain liquid water, but the conditions are right for oceans, rivers, and life.
Future Plans for Observation
On the TRAPPIST-1 website, there are a few plans and next steps listed. The first will be to utilize the Kepler telescope to search for additional planets around the star. Furthermore, astronomers will use light curves to attempt to find more accurate measurements of these planets’ masses. A more accurate measurement of mass will help determine whether these planets are gaseous or rocky.
From there, astronomers will begin to analyze the exoplanet’s atmospheres. It sounds unlikely, but astronomers can determine the atmospheres of far flung planets by analyzing the light coming from the star. Inevitably, some of the light that shines from the star will pass through the atmosphere of its planets before reaching Earth. The elements in the atmosphere will actually have an impact on the light that can be detected and measured.
In only 25 years of confirmed exoplanet findings, astronomers have been able to locate 3,586 exoplanets in 2,691 star systems. It is only a matter of time before we can confirm that one of these systems has an oxygen-rich atmosphere with liquid water, and maybe even biological life.
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