PART V · First Room · Observational Astronomy and Life

Exoplanet Hunting — Finding an Unseen Planet
in the Faint Flicker of Starlight

What if every single star in the night sky were a sun with planets of its own? We cannot see those planets directly. They are far too dim, buried in the glare of their star. And yet over the past 30 years, more than 5,000 exoplanets have been found. The trick is not to catch the planet itself, but to read the faint trace a planet leaves on its star.

§1There Are Planets Beyond the Solar System

Throughout the 20th century, the only planets we knew were the eight of our own Solar System. We suspected other stars had planets too, but there was no proof. Then in 1995, the Swiss astronomers Michel Mayor and Didier Queloz discovered 51 Pegasi b, a planet orbiting the star 51 Pegasi — the first confirmed exoplanet orbiting a Sun-like star. It was a Jupiter-sized giant gas planet hugging its star so tightly that it circles it in just four days, a surprise found nowhere in our Solar System.

In plain words Seeing an exoplanet directly is like trying to spot a single firefly sitting next to a lighthouse hundreds of km away. It is far too dim and completely lost in the lighthouse's beam. So instead of the firefly, astronomers catch the lighthouse beam dimming ever so slightly, or wobbling — when the firefly passes in front of it, or tugs on the lighthouse just the tiniest bit.

§2The First Hunting Method — Transit

When a planet crosses in front of its star along our line of sight, part of the starlight is briefly blocked and the star dims very slightly. Plotting this tiny drop in brightness over time gives a light curve, and each time the planet passes, a regular U-shaped "dip" appears in the curve. When an Earth-sized planet crosses in front of the Sun, the starlight dims by only about 0.01% — catching that faint flicker is the transit method. How deep the dip is tells you how large the planet is compared to the star, and how often the dip repeats tells you the orbital period.

§3The Second Hunting Method — Radial Velocity (Doppler)

A planet does not simply orbit its star; in fact the star and planet both orbit their common center of mass. The heavier star moves only a little, but move it does. When the star comes toward us, its light spectrum is pushed toward shorter wavelengths (blueshift); when it moves away, toward longer wavelengths (redshift), periodically. From the size of the sine wave traced by this wobble velocity (radial velocity), we estimate the planet's minimum mass. 51 Pegasi b was found by exactly this method. If the transit gives a planet's size, radial velocity gives its mass — combine the two and you get the density, that is, whether it is rock or gas.

§4Where Life Might Live, and Reading the Atmosphere

A discovered planet draws special attention if it lies at a distance from its star that is just warm enough for liquid water to exist — neither too hot nor too cold, the habitable zone. The Kepler telescope (2009) and TESS (2018) monitored vast numbers of stars at once and found more than 5,000 planets by transit. Going further, the James Webb Space Telescope (JWST) splits into a spectrum the starlight that passes through a planet's atmosphere during a transit, reading which molecules (water, carbon dioxide, and so on) are present in that atmosphere. If a combination of gases that could only be explained by biological activity (a biosignature) were found, it would be a historic event — but there is no such confirmation yet.

What Have We Really "Seen"?

Most exoplanets found so far were not seen directly in a photograph. What we observed is the indirect evidence a planet leaves on its star — the faint flicker (transit) or wobble (Doppler) of starlight. In its ability to "detect the unseen by its shadow," exoplanet hunting is a prime example of just how refined astronomy has become.

Key points

  • Exoplanet = a planet beyond the Solar System, orbiting another star (51 Pegasi b was the first confirmed, in 1995)
  • Transit: the tiny drop in brightness (the dip in the light curve) as a planet crosses in front of its star → planet size and period
  • Radial velocity (Doppler): the blueshift and redshift from the star's wobble → planet minimum mass
  • Habitable zone: the orbital band warm enough for liquid water to exist
  • More than 5,000 confirmed by Kepler and TESS, with JWST searching for biosignatures via atmospheric spectroscopy (no life found yet)
EXP.15 — Transit Light CurveDepth —
0.11 R★
Medium
Mini goal — infer planet size from the curve depth As a small planet crosses in front of the star above, the light curve below carves out a U-shape. The depth of the dip is exactly (Rp/R★)² — double the planet's size and the dip gets four times deeper. Change the size with the slider and watch how the depth value changes, then work it the other way: "the dip is this deep, so the planet is what % of the star."
EXP.15b — Radial Velocity (Doppler) Wobble
A planet does not orbit its star alone; the star and planet both orbit their common center of mass. The heavier the planet, the more it wobbles the star. As the star approaches (blue) and recedes (red) the spectral lines shift, and the amplitude of the radial-velocity curve (sine wave) below grows — observe it.
RADIAL VELOCITYVelocity amplitude —
Moderate
Observe — Increase the planet's mass and both the star's wobble (its circular orbit) and the amplitude of the radial-velocity sine wave grow together. When the star comes toward us, the star and its spectral lines shift blue (#7fd8e6); when it moves away, red (#e07a6b). A very light planet barely wobbles the star, so check that its signal is faint too.
DEEP DIVE — Equations & History
Transit Depth — The Planet's SizeTransit Method
$\frac{\Delta F}{F} = \left(\frac{R_p}{R_\star}\right)^2$
The fraction by which the starlight drops, ΔF/F, equals the ratio of areas between the planet's disk and the star's disk. That is, the square of the radius ratio. Jupiter (about 1/10 the Sun's size) dims the star by about 1%, Earth (about 1/109) by only about 0.01% — catching that small number is the precision of transit observation.
Doppler Shift — The Star's WobbleRadial Velocity
$\frac{\Delta\lambda}{\lambda} = \frac{v}{c}$
How far a spectral line's wavelength is shifted, by Δλ, equals the star's radial velocity v divided by the speed of light c. Jupiter wobbles the Sun at about 12 m per second — this faint wobble, roughly walking speed, is read from a spectrum hundreds of light-years away. From the size of v we estimate the planet's minimum mass.
HISTORY — Exoplanet Exploration Timeline
1995
Mayor and Queloz discover 51 Pegasi b — the first exoplanet of a Sun-like star (2019 Nobel Prize in Physics)
2009
Kepler Space Telescope launched — finds thousands of planet candidates in bulk by transit
2018
TESS launched — sweeps the bright, nearby stars across the whole sky hunting transit planets
2022~
James Webb Space Telescope begins spectroscopic observation of exoplanet atmospheres (detecting water, CO₂, and more)
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