How does one detect a planet-sized object orbiting a star dozens of lightyears away? The magnitude of the task becomes clear if we consider that even stars appear as nothing more than pinpoints of light when viewed with even the largest telescopes. Planets have only a fraction of the mass of a star, and as a result the nuclear fusion reaction that makes stars “burn” does not take place. Planets, as a result, are very small and very dark compared to stars, which in itself would make them very difficult to detect from Earth. Add to that the fact that these inconspicuous objects are inevitably found right next to the stars they orbit, and the task of observing them becomes well nigh impossible.
Astronomers, however, are nothing if not ingenious. Since the planets cannot be observed directly, the planet hunters decided instead to observe stars, and look for the minute effects that orbiting planets can have upon them. Astronomers have been looking for some these effects since the dawn of the 20th century, but only in the past ten years have instruments become sensitive enough to finally detect them without ambiguity.
How are scientists searching for extrasolar planets? Read here of the different approaches, their strengths and weaknesses, and how planet hunters are pursuing them all.
Up until the launch of the planet hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. The vast majority of Exoplanets detected from Earth were discovered by this method.
This method detects distant planets by measuring the minute dimming of a star as an orbiting planet passes between it and the Earth. The passage of a planet between a star and the Earth is called a "transit." If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period.
Microlensing is the only known method capable of discovering planets at truly great distances from the Earth. Whereas radial velocity searches look for planets in our immediate galactic neighborhood, up to 100 light years from Earth, and transit photometry can potentially detect planets at a distance of hundreds of light-years, microlensing can find planets orbiting stars near the center of the galaxy, thousands of light-years away.
Astrometry is the science (and art!) of precision measurement of stars' locations in the sky. When planet hunters use astrometry, they look for a minute but regular wobble in a star's position. If such a periodic shift is detected, it is almost certain that the star is being orbited by a companion planet.
Direct imaging of exoplanets is extremely difficult, and in most cases impossible. Being small and dim planets are easily lost in the brilliant glare of the giant stars they orbit. Nevertheless, even with existing telescope technology there are special circumstances in which a planet can be directly observed.
Detecting complex life on distant planets, such as plants, is difficult, and for that scientists will need to rely on subtle indicators such as the “red edge.” How should we look for these signs of complex life on distant planets, and how likely are we to find them?
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