Planet Hunter
A telescope has been staring at a star and the star keeps dimming. Read the light curve: the depth of the dip is the planet's size, the gap between dips is its year, and the year tells you whether it is hot, cold or in the habitable zone. Five planets per run, against the clock. Get all five with no mistakes and no peeking in under four minutes and the mission is done.
The light curve
What the telescope saw: the star's brightness, night after nightHow big is the planet?
Read the dip, then pick an answer to start the clock.
Field guide
Size: the dip is (planet radius ÷ star radius)², so the same planet makes a dip four times deeper in front of a red dwarf half the Sun's width. Year: the time from one dip to the next. Habitable zone: the year where a planet gets Earth's sunshine: about 365 days round a Sun-like star and about 47 days round the red dwarf (0.041 L☉, 0.5 M☉); a year within about 30 % of that counts as "just right", shorter is hotter, longer is colder. Reading this during a run counts as peeking, so that run will not set a best time.
What you're learning
How most of the 5,000 known exoplanets were actually foundThe transit method. When a planet passes in front of its star, the star dims by the fraction of its disc the planet covers: depth = (Rplanet ÷ Rstar)².
- A Jupiter in front of a Sun-like star blocks about 1 % of the light. An Earth blocks 0.0084 %, one part in twelve thousand, which is why finding Earths needs a space telescope.
- The star matters: the same Earth-sized planet makes a dip four times deeper in front of a red dwarf half the Sun's width. Read the star before you read the dip.
- The dips repeat once an orbit, so the gap between them is the planet's year. Short years mean small, hot orbits: a 10-day year round a Sun-like star is only 0.09 AU out, a quarter of Mercury's distance.
- The habitable zone is the distance where a planet gets Earth's sunshine. Round a dim red dwarf it is close in, so a habitable-zone year there is only about seven weeks.
- Want the physics behind each number, and to build your own light curves? The Exoplanets lesson has it, and Kepler's Laws turns a year into an orbit.