Step by Step
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Discovery history
The first confirmed exoplanets were found in 1992 (orbiting a pulsar) and 1995 (51 Pegasi b, orbiting a Sun-like star — work that earned a Nobel Prize in 2019). The Kepler Space Telescope alone found over 2,600 confirmed exoplanets, revealing that most planets exist in multi-planet systems.
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Four detection methods
Transit method: measures the dip in a star's brightness as a planet passes in front of it, revealing the planet's size (used by Kepler and TESS). Radial velocity: measures the Doppler wobble of a star caused by an orbiting planet's gravity, revealing the planet's mass. Direct imaging: works mainly for large, young planets far from their star. Microlensing: detects planets via single, non-repeating gravitational lensing events.
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What we've learned about planet demographics
"Hot Jupiters" (gas giants orbiting extremely close to their star) were an early surprise finding. Super-Earths and small rocky planets have both turned out to be quite common throughout the galaxy.
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The habitable zone and modern discoveries
The habitable zone (HZ) is the range of distance from a star where liquid water could exist on a planet's surface. TRAPPIST-1 is a notable system with 7 rocky planets, 3 of which lie within the habitable zone. The James Webb Space Telescope (JWST) is now used for atmospheric characterization of exoplanets — it has already detected CO₂ in some exoplanet atmospheres.
Applied Walkthrough
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Astronomers using the transit method watch a distant star's brightness dip slightly as an orbiting planet passes in front of it — this technique, used by missions like Kepler and TESS, reveals the planet's size based on how much light it blocks.
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A different team instead uses the radial velocity method, measuring the tiny gravitational wobble a planet induces in its host star — this reveals the planet's mass rather than its size.
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Among the thousands of exoplanets discovered this way, the TRAPPIST-1 system stands out: 7 rocky planets orbit this star, with 3 of them located within the habitable zone where liquid water could potentially exist.
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The James Webb Space Telescope is now examining planets like these in even more detail, using atmospheric characterization to detect specific molecules — including CO₂ — in some exoplanet atmospheres, a major step toward eventually identifying potentially habitable worlds.
Exam Application
Exams test whether you can match each exoplanet detection method (transit, radial velocity, direct imaging, microlensing) to what it actually measures (size versus mass, for example), and whether you know key discoveries like 51 Pegasi b and TRAPPIST-1.
⚠ Common Trap
The most common trap is confusing what the transit method measures versus what radial velocity measures — transit reveals a planet's SIZE (from the brightness dip), while radial velocity reveals a planet's MASS (from the star's gravitational wobble) — these are different physical properties, not interchangeable measurements.
✓ Quick Self-Check
1. What was the first exoplanet confirmed around a Sun-like star, and when was it discovered?
51 Pegasi b, discovered in 1995.
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2. What does the transit method measure, and what does it reveal about a planet?
The dip in a star's brightness as a planet passes in front of it; it reveals the planet's size.
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3. What does the radial velocity method measure, and what does it reveal?
The Doppler wobble of a star caused by an orbiting planet's gravity; it reveals the planet's mass.
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4. What is the habitable zone?
The range of distance from a star where liquid water could exist on a planet's surface.
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5. What is notable about the TRAPPIST-1 system?
It has 7 rocky planets, 3 of which lie within the habitable zone.
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