Why it's so hard to find the next Earth, even if you're looking right at it

With transit, scientists watch a star, measure its brightness and wait for that brightness to dim as a planet passes in front of it. It’s different from the radial velocity, or Doppler, technique — which was used to find Proxima Centauri b. There, scientists are looking for changes in the spectrum of light caused by interactions between the gravity of the star and the gravity of a nearby planet.

Both of these systems have limitations, scientists told me. The transit method can find so many more planets because it’s tracking only a single measurement: star brightness, said Jessie Christiansen, staff scientist at the NASA Exoplanet Science Institute. Kepler, the famous space-based telescope, can track brightness on 200,000 stars at once, she said. But transit can only spot planets that happen to geometrically line up in just the right way so that we see their star dim when they pass between it and Earth.

Radial velocity, meanwhile, is much more specific. Scientists separate the different wavelengths of light from stars so they can track how the spectrum changes and see the photonic echo of slight wobbles in the solar orbits. That means it’s harder to study faint, far-away stars — less of their light reaches us, to begin with — and scientists can look at only one star at a time, drastically slowing the rate of detection compared with transit. But radial velocity doesn’t depend on geometric good fortune, so it can see planets that transit never would. “That’s partly why people are so excited about the Proxima Centauri detection,” said Sara Seager, professor of planetary science and physics at MIT. “Radial velocity can find those, whereas if you were using transit technique, you might miss it because it wasn’t lined up.”