Astronomy · Rung 10 of 12

Exoplanets

There are more than seven thousand confirmed planets outside the Solar System, and almost none has been seen: they are deduced. The transit method measures the dip in brightness as the planet crosses its star, and the depth of that dip gives the size; radial velocity measures the star's wobble as the planet tugs it, and that gives the mass. With both you get density, and with density you know whether it is rock or gas. The habitable zone is where water can be liquid on the surface, no more and no less: it does not mean inhabited, nor even habitable, because you also need an atmosphere, a magnetic field and chemistry.

Where this sits

Comes after
The life of a star

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What was the surprise about the first exoplanet around a Sun-like star, in 1995?

    • That it had moons
    • That it was rocky
    • That it lay in the habitable zone
    • That it was a giant orbiting in four days

    Why 51 Pegasi b: half a Jupiter at under a twentieth of the Earth-Sun distance, with a 4.2-day year. Nobody expected it, because on the formation model a giant cannot be born that close in. The answer was planetary migration: it forms far out and spirals inwards through friction with the disc. Mayor and Queloz took the Nobel in 2019.

    easier

  2. The transit gives the size. Which method gives the mass?

    • Radial velocity: the star's wobble
    • The star's colour
    • The total brightness of the system
    • Galactic astrometry

    Why A planet does not orbit its star: both orbit their common centre of mass, so the star wobbles. That to-and-fro shifts its spectrum by the Doppler effect, and the amplitude gives the mass. With size and mass you have density, and density tells you rock or gas. That is why systems with both measurements are the valuable ones.

    in between

  3. The first exoplanets found were almost all hot Jupiters. Why?

    • They are the easiest to detect, not the most common
    • They formed earlier
    • They are the most abundant in the galaxy
    • They are the only stable ones

    Why Pure selection bias. A large, very close planet blocks a lot of light, tugs hard at its star and repeats its transit every few days: it is what any method finds first. Small, distant ones like Earth give tiny signals and you must wait years to see two transits. Mistaking “the first thing we see” for “what there is most of” is a classic error.

    harder