What Are Exoplanets?
Exoplanets are planets orbiting stars other than our Sun. Until 1992, we didn't know for certain that any existed. Now we've confirmed over 5,500—and estimate billions in our galaxy alone.
The Discovery
- Found around a pulsar (dead star)
- Surprising location
- Detected through timing of pulses
- 51 Pegasi b
- "Hot Jupiter"—gas giant orbiting very close to star
- Won 2019 Nobel Prize for discoverers
How We Find Them
- Planet passes in front of star
- Slightly dims the star's light
- Can determine size and orbit
- Most successful method (Kepler, TESS missions)
- Star wobbles from planet's gravity
- Doppler shift in star's light
- Reveals planet mass and orbit
- Actually photographing the planet
- Very difficult—stars are much brighter
- Works best for large planets far from stars
- Planet's gravity bends light from background star
- One-time events
- Can detect planets very far away
What We've Found
- Gas giants orbiting very close to stars
- Surface temperatures over 1000°C
- Unexpected—not predicted by our solar system
- Larger than Earth but smaller than Neptune
- No equivalent in our solar system
- Very common
- Small gas planets
- Probably have thick atmospheres
- Also very common
- Rocky planets in habitable zones
- Where liquid water could exist
- Several candidates found
The Habitable Zone
- Not too hot (too close to star)
- Not too cold (too far from star)
- "Just right" for life as we know it
Many potentially habitable planets have been found, including in the TRAPPIST-1 system (7 planets, several in habitable zone).
The Search for Life
- Looking for biosignatures
- Oxygen, methane, other gases life might produce
- No definitive detections yet
Big Questions
- How common are Earth-like planets?
- How common is life?
- Are we alone?
With billions of planets in our galaxy, the possibilities are staggering.