Planet Hunting : How Earth-like worlds beyond our solar system are discovered ?
- Clark Newman
- Aug 5, 2018
- 3 min read
This article first appeared in our issue 02 Nov/Dec 2017
Article text:
We are in the middle of a planet-finding bonanza. In just the past five years, we’ve discovered more planets outside our solar system than in the rest of history combined! And as for finding more planets, the sky is literally the limit! What is an exoplanet and how do we find one?
An exoplanet is simply any planet discovered outside of our solar system. And the simplest method to detect a planet? Direct Imaging: Point a very powerful telescope toward a star and view the planet(s) orbiting that star. But even with the most powerful telescopes we have, we can only directly image huge planets (larger than Jupiter). And those planets would have to be close enough to a star bright enough to reflect light for a telescope to see a direct image.
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Even finding planets within our own solar system is no simple task. Only Mercury, Venus, Mars, Jupiter, and Saturn have been seen with the naked eye, and they have been known since ancient times. In fact, we started discovering Jupiter’s moons before we finished discovering every planet in our own solar system. Jupiter’s first four moons (Io, Europa, Ganymede, and Callisto) were all discovered in 1610, but Uranus wasn’t discovered until 1781. Pluto was discovered in 1930 and then unceremoniously demoted to a dwarf planet in 2006. By then we had well begun identifying planets beyond our solar system.
That’s great, but we’re more interested in smaller, Earth-like planets. Why? Because they could exhibit features like our planet and even possibly harbor life. Finding evidence that a planet outside our system harbors life is the holy grail of planet hunting! Right now, we are finding plenty of planets slightly larger than ours that are called Super Earths. Super Earths cannot be seen directly, but we know they are there. If we can’t see a Super Earth, how can we detect it?
The transit method marks the latest fashion in planet detection, big and small. Direct imaging detects the light reflected off of a planet, but the transit method detects the absence of light from the star when a planet crosses in front of it. If the alignment is right, a planet orbiting in front of a star will temporarily decrease that star’s brightness in a predictable fashion. If the star’s brightness is graphed over time, the size and shape of the dip in brightness will give us information about the planet’s size and orbit.
In February 2017, NASA announced the record-breaking discovery of seven exoplanets at once! The transit method was used to detect the seven planets of the TRAPPIST-1 system. All seven planets are roughly Earth-sized or smaller, and three of them fall within their parent star’s habitable zone. That zone refers to the distance at which a planet has the right temperature, given the star’s heat, to hold liquid water. The parent star in this case is a red dwarf, which is a smaller and cooler star than our own Sun. So, the habitable zone is much closer to the star. TRAPPIST-1’s parent star is only 8% of the mass and 11% of the diameter of our Sun.
TRAPPIST-1 is “only” 39.5 light years away. This suggests that solar systems like ours are fairly common, that planets are everywhere, and improves the chances that life exists elsewhere in our galaxy and universe. More and more powerful telescopes continue to come online, techniques are becoming more refined, and there are even some methods to detect the composition of exoplanet atmospheres coming around. It is a very exciting time for planet hunters. Perhaps you’ll find one, or even a whole system of them!
What is a Light Year?
It is the distance that light travels in one year.
Light travels 186,000 miles in a second (300,000 kilometers per second).
So, in a year, light will travel -
186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year (9,460,800,000,000 kilometers). WOW!
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