🪐 Exoplanets: Worlds Beyond

What you’ll learn

1. A Sky Full of PlanetsUnderstand what exoplanets are, when we first found them, and why detecting them is so difficult.Until the 1990s no planets were known beyond the Sun; the 1995 discovery of 51 Pegasi b opened the floodgates, and over 5,000 exoplanets are now confirmed. Because stars vastly outshine their planets, astronomers rely on indirect methods rather than direct images. The discovered worlds span a huge range and look very different from our own solar system.
2. Catching a Shadow: The Transit MethodLearn how the transit method works and how Kepler used it to reveal a galaxy full of planets.The transit method detects a planet by the tiny, repeating dip in starlight as it crosses its star, with the dip's depth giving the planet's size and its spacing giving the orbit. Light curves must be checked against impostors. NASA's Kepler stared at over 150,000 stars and found more than 2,600 planets, showing small rocky worlds are common.
3. Catching a Wobble: Radial VelocityUnderstand the radial velocity method, how it weighs planets, and the precision it demands.The radial velocity method detects a planet from the Doppler wobble its gravity induces in its star, and it measures the planet's mass — complementing the transit method's size to yield density. The technique found the first Sun-like-star planet and requires ultra-stable spectrographs able to sense shifts of centimeters per second.
4. The Zoo of Strange WorldsSurvey the diversity of exoplanets, from hot Jupiters and super-Earths to TRAPPIST-1 and rogue worlds.Exoplanets include hot Jupiters that migrated inward, super-Earths that are the galaxy's most common planet yet absent from our system, and TRAPPIST-1's seven Earth-sized worlds. Stranger still are lava planets, possible water worlds, and rogue planets wandering starless — each stretching theories of how planetary systems form.
5. The Habitable Zone, HonestlyUnderstand the habitable zone, its genuine limits, and the special challenges of red dwarf systems.The habitable zone is the band around a star where surface liquid water is possible, a useful first filter. But it guarantees nothing: Venus and Mars border our zone yet are a furnace and a frozen desert due to their atmospheres. Many nearby habitable-zone planets orbit red dwarfs, which bring tidal locking and dangerous flares.
6. Reading Alien AtmospheresLearn how transmission spectroscopy and JWST reveal exoplanet atmospheres and the careful hunt for biosignatures.Transmission spectroscopy reads an atmosphere from the starlight that filters through it during a transit, imprinting absorption lines of its gases. The James Webb Space Telescope has detected carbon dioxide, water, and more on distant worlds. The ultimate goal is a biosignature, but any claim demands ruling out non-living explanations.
7. Are There Other Earths?Understand what the statistics suggest about Earth-like worlds, the nearest candidates, and the next telescopes.Kepler's statistics suggest a few percent to a fifth of Sun-like stars host an Earth-sized habitable-zone planet, implying billions in the galaxy. Nearby candidates include Proxima Centauri b and the TRAPPIST-1 worlds, though none is shown habitable. New missions like PLATO and giant telescopes aim to characterize true Earth analogs and search for life.

Questions this course answers

Grounded in trusted sources

• NASA Exoplanet Science Institute
• European Space Agency
• NASA Jet Propulsion Laboratory
• Space Telescope Science Institute

Every Wunder lesson is built from real, reputable sources — never invented.

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