Astronomy & spaceflight
Exoplanets
Distant worlds around other stars: how a tiny shadow and a slight wobble revealed more than 6000 planets, and what the habitable zone really means.
For a long time we knew planets only from a single solar system, our own. Whether other stars are circled by worlds too was an open question people could argue about at length. Since 1995 we know better: planets are the rule, not the exception. By now more than 6000 planets around other stars have been confirmed, and new ones turn up almost every week. These worlds are called exoplanets.
Why we hardly ever see them
A planet does not shine by itself, it only throws back a little starlight. Right next to it sits its star, shining billions of times brighter. From many away that is like spotting a gnat right beside a floodlight. So researchers usually do not point their telescopes at the planet at all. They watch the star and hunt for the tiny traces a planet leaves there.
1 AU = distance from Earth to the Sun ≈ 150 million km.
Two traces a planet leaves
The first trace is a shadow. When a planet passes exactly in front of its star as seen from here, it covers a tiny part of the star's disc, and the measured brightness drops for a few hours. This is called a transit. For a giant planet like Jupiter the drop would be about one percent, for an Earth in front of our Sun less than a hundredth of a percent. Because the planet keeps travelling the same , the dip repeats in a fixed rhythm. The rhythm reveals the orbital period, the depth reveals the size of the planet. The space telescopes Kepler and TESS have tracked down thousands of planets this way.
The second trace is a wobble. Strictly speaking a planet does not orbit a star that stays put: the two of them circle their common centre of mass. So the star runs a small loop of its own, now a little towards us, now a little away. As it does, its light shifts measurably, much as the tone of a passing ambulance shifts. That is exactly how Michel Mayor and Didier Queloz found the first planet around a Sun-like star in 1995, 51 Pegasi b, which won them the Nobel Prize in Physics in 2019. The transit reveals the size, the wobble reveals the mass. Both together give the , and only that tells you whether a planet is made of rock or of gas: rock is about five times as dense as water, a gas planet barely denser.
The habitable zone
The most exciting is the band of distances around a star where a planet's surface neither boils nor freezes for good, so that liquid water could survive. It is called the habitable zone. Around our Sun, Earth sits in the middle of it, Venus is too close and far too hot, and Mars circles the cold outer edge. Cooler stars shine more weakly, so their habitable zone lies much further in: Proxima Centauri b, the nearest known exoplanet at 4.2 light years, needs only 11 days for one orbit there. A word of caution: habitable means possible to live on, not inhabited. Whether there is water, air or even life has to be shown planet by planet, for instance when a little starlight shines through an atmosphere during a transit.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What is an exoplanet?
Why can exoplanets almost never be photographed directly?
During the transit of a giant planet, its star's brightness drops by 1 percent. How many percent of the starlight still reaches us in the meantime?
When a planet passes in front of its star, the star becomes … for a few hours.
An exoplanet orbits its star in 12 days. A team wants to observe three transits before announcing the discovery. At least how many days lie between the first and the third transit?
Match each term from exoplanet research with the description that fits.