Exoplanets & Life

Thirty years ago the number of confirmed planets outside the solar system was zero. It is now over five thousand, and the field has moved from discovery to characterisation. Two methods found most of them: transits, where a planet crossing its star dims it by a fraction of a percent, and radial velocity, where the planet's gravity makes the star wobble by metres per second. Between them they give size and mass, and therefore density — the difference between a rock and a gas ball.

Both methods are strongly biased, and the catalogue is a portrait of the bias as much as of the galaxy. A transit requires the orbit to be edge-on from here, which for an Earth-like orbit is about a half-percent chance, and it repeats on the orbital period — so a survey that runs for four years finds planets with years shorter than four years, and Jupiter analogues almost by accident. Radial velocity favours heavy planets close in, for the same reason a heavier child on a roundabout is easier to notice. Correcting for all of that is how the field arrives at statements about how common planets are, and those corrections are where the arguments live. The genuine surprise survived them: most systems look nothing like ours. Hot Jupiters orbit in days, and sub-Neptunes — the single most common planet type in the galaxy — have no local example at all.

The current frontier is atmospheres, read from starlight filtered through them during a transit, which is what Webb was built for. It is a hard measurement: the signal is a few parts in ten thousand, and the star's own spots and flares imprint themselves on it. Webb has cleanly detected carbon dioxide, sulphur dioxide and evidence of photochemistry on hot giant planets, which is real progress. It has also produced the field's most public argument, over whether a possible biosignature on a sub-Neptune is a molecule or a data-reduction choice. That is the honest state of things: the instrument is good enough to start the conversation about life and not yet good enough to end one.