NASA uses a variety of sophisticated techniques to detect planets beyond our solar system (exoplanets), with the most prominent being the transit method.
The Transit Method: Spotting Shadows
The transit method relies on observing the slight dimming of a star's light when a planet passes in front of it, from our perspective. Imagine a tiny insect crawling across a spotlight – the light will briefly dim. This dimming is proportional to the planet's size relative to the star.
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How it Works:
- A telescope, often space-based (like the Kepler Space Telescope or TESS), continuously monitors the brightness of thousands of stars.
- Astronomers look for periodic dips in a star's light curve. These dips indicate a possible transit.
- The depth of the dip reveals the planet's size (larger planets cause deeper dips).
- The time between dips reveals the planet's orbital period (how long it takes to orbit the star).
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Example: Kepler-186f, one of the first Earth-sized planets found in the habitable zone of another star, was discovered using the transit method.
Transit Spectroscopy: Analyzing Planetary Atmospheres
A related technique, transit spectroscopy, analyzes the starlight that filters through a planet's atmosphere during a transit. Different elements and molecules in the atmosphere absorb specific wavelengths of light. By analyzing the spectrum of light that reaches us, scientists can identify the atmospheric composition of the exoplanet. This is the technique referenced by the short answer: "when light from a star travels through the atmosphere of an orbiting planet and reaches our telescopes – in space or on the ground – and tells about where it's been."
Other Planet Detection Methods
While the transit method is the most successful, NASA also uses other methods:
- Radial Velocity (Doppler Spectroscopy): This method detects the wobble of a star caused by the gravitational pull of an orbiting planet. The star doesn't remain perfectly still; it moves slightly in a small circle or ellipse, which can be detected by shifts in the star's light spectrum.
- Direct Imaging: This involves directly taking a picture of a planet. This is extremely challenging because planets are much fainter than their host stars. It requires blocking the star's light using a coronagraph and advanced image processing techniques. This works best for large, bright planets orbiting far from their stars.
- Microlensing: This method uses the gravity of a star and its planet to bend and magnify the light from a more distant background star. The amount of magnification and the duration of the event can reveal the presence and properties of the planet.
Combining Methods for Better Understanding
Often, NASA uses multiple detection methods to confirm the existence of a planet and to learn more about its properties, such as its mass, radius, density, and atmospheric composition. For example, a planet discovered through the transit method might have its mass confirmed using the radial velocity method.
