Finding exoplanets was only the beginning. Once astronomers could measure orbits, masses, and sizes, a deeper question emerged:
What are these planets actually like?
Do they have atmospheres? Clouds? Winds? Oceans? Ice? Lava fields? Could any of them support liquid water — or even life?
This section explores how astronomers now probe the physical nature of exoplanets. You’ll see how we have moved from counting planets to characterizing worlds.
Transit Spectroscopy: Reading Planet Atmospheres
When a planet passes in front of its star, most of the star’s light continues through unchanged. But a small fraction passes through the planet’s atmosphere before reaching Earth.
Gases in that atmosphere absorb particular wavelengths of light, leaving behind tiny fingerprints in the star’s spectrum. This method is called transit spectroscopy.
By comparing the star’s spectrum before and during a transit, astronomers can identify atmospheric molecules such as:
- water vapour (H2O),
- carbon dioxide (CO2),
- carbon monoxide (CO),
- methane (CH4),
- sodium and potassium,
- and aerosols such as clouds and hazes.
These measurements do not produce pictures. Instead, they reveal a planet’s chemical composition through subtle changes in light.
Emission Spectroscopy and Phase Curves
Planets also emit their own radiation, especially in infrared wavelengths. When a planet passes behind its star, astronomers can measure the tiny drop in total light due to the loss of the planet’s thermal emission. This method is known as secondary eclipse spectroscopy.
By measuring light over an entire orbit, it is also possible to reconstruct how temperature changes across a planet’s surface — a technique called phase curve analysis.
These tools allow astronomers to:
- map day–night temperature differences,
- detect atmospheric circulation patterns,
- infer jet streams and wind speeds exceeding kilometres per second,
- estimate reflectivity (albedo),
- and search for cloud layers.
In some cases, astronomers can now tell whether a planet has a global heat-redistribution system or a permanent dayside inferno and nightside freeze.
JWST: Exoplanets in the Infrared Age
The James Webb Space Telescope (JWST) represents a revolution in exoplanet science. For the first time, astronomers can study exoplanet atmospheres with enough precision to detect key molecules directly.
JWST observations have already:
- measured carbon dioxide in exoplanet atmospheres,
- detected water vapour in multiple worlds,
- identified clouds and hazes,
- and measured vertical temperature profiles.
One landmark observation involved the planet WASP-39 b, where JWST detected carbon dioxide with extraordinary clarity — demonstrating that exoplanet chemistry can now be studied reliably rather than speculatively.
This marks the beginning of a new era: worlds beyond the Solar System are now laboratories, not just orbiting points of light.
From Mass and Radius to Interior Structure
Detecting atmospheres is only part of the story. By combining different observation methods, astronomers can infer what planets are made of inside as well.
When a planet’s:
- radius (from transits), and
- mass (from radial velocity or astrometry),
are known together, its density can be calculated.
Density provides clues about composition:
- Low density suggests gas giants or thick atmospheres.
- High density suggests rocky or metallic interiors.
- Intermediate values indicate ocean worlds, gas-enveloped super-Earths, or ice-rich planets.
Interior models then allow astronomers to estimate:
- core sizes,
- mantle structure,
- water inventories,
- and atmospheric thickness.
Planets now have inferred geologies — not just orbits.
Habitability: What Makes a World “Earth-like”?
A planet’s location is not enough to determine whether it could support life.
The so-called habitable zone simply describes where liquid water could exist on the surface — assuming Earth-like conditions. But habitability depends on many factors:
- atmospheric pressure,
- composition,
- cloud cover,
- rotation rate,
- magnetic fields,
- geological activity, and
- stellar behaviour.
A planet may sit in the habitable zone but be sterile. Others may lie outside it and still host subsurface oceans or exotic chemistry.
Habitability is therefore not a location — it is a set of conditions.
Searching for Biosignatures
Eventually, astronomers aim to detect not just atmospheres, but signs of life.
Possible biosignatures include:
- oxygen and ozone,
- methane in disequilibrium,
- nitrous oxide,
- certain combinations of gases that cannot persist together without replenishment.
The key idea is chemical imbalance: life drives atmospheres away from equilibrium.
However, biosignatures are subtle. Many natural processes mimic biological signals. This is why astronomers emphasize caution, redundancy, and cross-validation with multiple methods.
Life will not be announced with certainty by a single molecule.
Upcoming Observatories
Several missions are poised to extend this work:
- ARIEL (ESA) — a mission dedicated almost entirely to exoplanet atmospheres.
- PLATO — a census mission focusing on Earth-sized worlds around nearby stars.
- Roman Space Telescope — equipped for microlensing and coronagraphy.
- Extremely Large Telescopes (ELT, TMT, GMT) — ground-based giants capable of atmospheric measurements.
Exoplanet research is now forecastable science, not heroic speculation.
The Big Shift
Thirty years ago, astronomers argued whether planets beyond the Solar System even existed.
Today, they are:
- measuring winds on distant worlds,
- detecting cloud decks,
- mapping temperatures,
- and cataloguing planetary chemistry.
Exoplanets are not imagined anymore.
They are measured.
And now, at last, we are moving from discovery… to understanding.
In the final section of this module, you will step back to survey where the field now stands — and where it is headed — as exoplanet science transitions from revolution to routine.
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