This lab works on a phone. On a bigger screen you can see the star, the light curve and the readouts at the same time.

Exoplanets

Nobody has ever seen most of these worlds. A planet crossing its star dims it by a fraction of a percent, and that is enough: the depth of the dip gives the planet's size, the gap between dips gives its year, and its year gives the size of its orbit. Then watch the star wobble, which is how the very first ones were found.

depth = (R_p ÷ R_s)² Kepler's third law The star wobbles The habitable zone

Pro

Catch a transit

Pick a star and a planet, set the orbit, and watch the light curve the telescope would see

The Sun Depth: 0.0084 % Year: 365 days The dip is exaggerated on screen · the numbers are not

Star

Planet

Planet radius 1.0 R⊕

Orbit size 1.00 AU

How central b = 0.30

Or load a real one

Readout

What the dip says

StarThe Sun PlanetEarth Planet radius6,371 km · 1.00 R⊕ Transit depth0.0084 % · 84 ppm Blocks one part in11,946 Transit lasts13.1 hours Year (from the gap)365 days Orbit size (Kepler)1.00 AU · 150 million km Chance it lines up0.47 % Habitable zone1.00 AU A year there365 days

What you're learning

A planet blocks a patch of its star's disc, so the fraction of light lost is the ratio of the two areas: depth = (R_p ÷ R_s)². Jupiter in front of the Sun takes away about 1 %. Earth takes 84 parts per million, one part in twelve thousand.

The dips repeat once every orbit, so the gap between dips is the planet's year. Kepler's third law then gives the size of the orbit: a³ = M × P² with a in AU, P in years and M the star's mass in Suns.

A transit only happens if the orbit is edge-on to us. The chance is roughly R_s ÷ a: for Earth around the Sun, about 1 in 200. The method misses almost everything, so every planet it finds stands for hundreds it cannot.

Reading the curve

Flat is the star on its own. The dip is the planet on the face of the star. The return to exactly the same level is how you know you saw a passing object and not a change in the star itself.

How long the dip lasts is how long the planet takes to cross, and the slanted edges are the planet sliding on and off the disc. A grazing planet, with b near 1, clips the edge and makes a shallow V; above about 1 it misses altogether.

The dip on screen is stretched so you can see it. A real Earth transit is invisible on this scale, which is why finding one needs a space telescope watching the same star for years.

Where could there be water?

The habitable zone is the distance where a planet gets about as much starlight per square metre as Earth does. Because brightness falls off as 1 ÷ distance², that distance is a = √(L ÷ L☉) AU, and the year there is T = √(a³ ÷ M) years.

Around a red dwarf with 4 % of the Sun's light, the habitable zone is only 0.2 AU out and a year is 47 days. Being that close makes a transit far more likely, which is why so much searching is aimed at small cool stars. Being in the zone does not make a planet habitable: it still needs an atmosphere, liquid water and a quiet star.

The star wobbles

A planet does not orbit a fixed star. Both circle their shared centre of mass, and the star's small circle shows up in its light

Jupiter around the Sun Star's speed: 12.5 m/s The star's circle is exaggerated on screen · the speed is not

Planet around the Sun

Readout

How fast the star moves

Planet's speed13,070 m/s Mass ratio M_p ÷ M_s9.54 × 10⁻⁴ Star's speed12.5 m/s Shift of a 500 nm line2.08 × 10⁻⁵ nm

Why the star moves at all

The planet pulls on the star exactly as hard as the star pulls on the planet. The star is much heavier, so it moves much less, but it moves: for a planet much lighter than its star, v_s = (M_p ÷ M_s) × v_p.

Jupiter makes the Sun move at 12.5 m/s, about walking pace. Earth makes it move at 8.9 cm/s, slower than a snail.

We see that speed as a Doppler shift: Δλ ÷ λ = v ÷ c. Jupiter's pull shifts a 500 nm line by two hundred-thousandths of a nanometre, one part in 24 million, which is why this takes a spectrograph built to stay stable for years.

Good to know

The first planet found around a Sun-like star, 51 Pegasi b in 1995, was found this way: a Jupiter-mass planet racing round its star every four days. A planet close in moves fast and tugs hard, so the wobble method finds massive planets on short, close orbits most easily. That is why the first exoplanets were hot Jupiters: not because they are typical, but because they shout loudest.

Put the two methods together and you get both the size (from the transit) and the mass (from the wobble) of the same planet, and from those its density: rock, ice or gas.

Practice

Answer a quick exoplanet question. Score counts answers right on the first try.

Challenge

Press New challenge.

Score: 0 Streak: 0 Attempted: 0

Choose the answer