How Scientists Study Distant Planets
No telescope can photograph the surface of a planet around another star. Astronomers learn what those worlds are made of anyway, from the shadows they cast and the light they filter.
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- What happened
- Since the first confirmed planets around other stars in the 1990s, astronomers have found more than 5,000, mostly by watching for the slight dimming when a planet crosses its star. The Webb telescope can now analyze starlight filtered through some of their atmospheres.
- Why it matters
- Planets are common; the question has become which ones resemble Earth and whether any show chemical signs of life. Answering that requires knowing what their atmospheres contain.
- What remains uncertain
- Atmospheric signals are faint and easily confused with stellar activity. Claims of specific molecules in small planets' atmospheres have been made and disputed. No detection of life has been established.
A planet around another star is lost in the glare of that star, which is typically a billion times brighter. Astronomers found a way around this that requires no image at all: they watch the star.
Transits
When a planet passes between its star and Earth, the star dims by a fraction of a percent for a few hours. The depth of the dip gives the planet's size; the interval between dips gives its orbit. Space telescopes designed for this purpose, first Kepler and then TESS, found thousands of planets by staring at stars and waiting.
Wobbles
A planet also tugs its star back and forth. The star's light shifts slightly toward blue as it approaches and red as it recedes. Measuring this gives the planet's mass. Combined with size from a transit, mass yields density, and density tells whether a world is rock, gas or something in between.
Atmospheres
During a transit, a sliver of starlight passes through the planet's atmosphere on its way to us. Molecules in that atmosphere absorb particular wavelengths. By comparing the star's spectrum during and outside the transit, astronomers can identify water, carbon dioxide, methane and other gases. Webb's infrared sensitivity made this possible for smaller, cooler planets for the first time.
The method has limits. Signals are tiny, stars are variable, and the analysis depends on models of what the atmosphere should look like. Several announced detections have been disputed by other teams working from the same data. The field has adopted a cautious vocabulary, distinguishing hints from detections and detections from confirmations.
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More than 5,000 planets around other stars have been confirmed.
Fact · confidence high
- NASA Exoplanet Archive · NASA / Caltech
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