Exoplanets are planets that orbit stars beyond our Sun, and their discovery has changed how scientists think about planetary systems. Most exoplanets are too small and dim to see directly beside the glare of their stars. Astronomers detect them by measuring tiny changes in starlight, stellar motion, or the planet's own emitted or reflected light.
These methods reveal planet size, orbit, mass, temperature, and sometimes atmospheric clues.
The most common discovery method is the transit method, which looks for a small dip in a star's brightness when a planet crosses in front of it. The radial velocity method finds planets by measuring how their gravity makes the star wobble toward and away from Earth. Direct imaging, gravitational microlensing, and astrometry add more ways to find planets in different kinds of orbits.
Combining methods gives a fuller picture because each method is sensitive to different planet sizes, distances, and orbital angles.
Key Facts
- Transit depth = (planet radius / star radius)^2
- Radial velocity detects Doppler shifts caused by a star moving toward and away from Earth.
- Orbital period can be found from repeated transits: period = time between matching brightness dips.
- Kepler's third law for a planet around a star: P^2 = a^3 when P is in years and a is in AU for a Sun-like star.
- Direct imaging works best for large, hot planets far from their stars where glare can be blocked.
- Microlensing occurs when gravity from a star and planet bends and magnifies light from a more distant background star.
Vocabulary
- Exoplanet
- An exoplanet is a planet that orbits a star outside our solar system.
- Transit
- A transit is the event in which a planet passes in front of its star and blocks a small fraction of the star's light.
- Radial velocity
- Radial velocity is the motion of a star toward or away from Earth, often measured using Doppler shifts in its spectrum.
- Doppler shift
- A Doppler shift is a change in the observed wavelength of light caused by motion between the source and the observer.
- Astrometry
- Astrometry is the precise measurement of a star's position in the sky to detect tiny wobbles caused by orbiting planets.
Common Mistakes to Avoid
- Confusing a transit dip with the planet giving off less light is wrong because the dip usually comes from the planet blocking part of the star's light.
- Assuming every planet around a star will transit is wrong because transits only occur when the orbit is aligned nearly edge-on from Earth's viewpoint.
- Using transit depth as the planet's mass is wrong because transit depth gives the planet's size relative to the star, not its mass.
- Ignoring repeated signals is wrong because a single brightness dip or velocity change may be noise, a starspot, or another object rather than a confirmed planet.
Practice Questions
- 1 A star's brightness drops by 1 percent during a transit. Using transit depth = (planet radius / star radius)^2, find the planet radius as a fraction of the star radius.
- 2 A planet transits its star on day 4, day 16, day 28, and day 40. What is the orbital period of the planet?
- 3 A large planet orbits very far from a young nearby star, while a small Earth-size planet orbits very close to a distant star. Which discovery method is more likely to find each planet, direct imaging or transit, and why?