Finding Exoplanets
Describe how astronomers find planets around other stars using recurring dips in starlight and spectral shifts caused by a star's motion toward or away from us. Explain the habitable zone and why atmospheric gases can suggest possible life without proving it.
What a learner can do afterwards
- Explains transit photometry: the small, periodic dip in a star's brightness when a planet passes in front of it
- Explains the habitable zone as a range of distances where surface liquid water could exist with suitable atmospheric conditions, without guaranteeing water or life
- Describes oxygen and methane as possible atmospheric clues to life, explains why substantial amounts together may need continuing sources, and states that non-living explanations must also be checked
1 · Read
Planets around stars other than the Sun are exoplanets. Some have been imaged directly, but many are found through effects on starlight. A transit happens when a planet crosses in front of its star from our viewpoint and blocks some light. Astronomers look for small dips that recur each orbit. Dust or measurement errors can also cause dips. Repeated dips need further checks too.
Continuous monitoring of fictional star Vesper finds matching dips beginning on days 2, 14 and 26, with no matching dips between them. These are successive dips. The gaps are 14 - 2 = 12 and 26 - 14 = 12 days. If one planet causes these transits, its orbit takes 12 days. Find a brightness drop by subtracting the during-transit reading from the usual reading: 500 - 496 = 4 units.
An orbiting planet's gravity pulls on its star. The radial velocity method measures its motion toward and away from us. Wavelength is the distance between neighbouring wave crests. A spectrum separates light by wavelength; absorbed wavelengths appear as dark lines. Motion toward us shifts these lines to shorter, bluer wavelengths; motion away shifts them to longer, redder wavelengths. Instruments measure tiny shifts; the whole star need not visibly change colour.
The habitable zone is a range of distances where a planet could have liquid water on its surface with a suitable atmosphere. Closer planets generally receive more energy from their star; farther planets receive less. The star's brightness affects where the zone lies. A planet in this zone is not guaranteed to have water, a suitable surface temperature or life.
A biosignature is a possible clue to life. Oxygen, methane and water vapour can also come from non-living processes. Chemical reactions can remove oxygen and methane. Substantial amounts of both together may need continuing sources. Life could supply them, but astronomers must also check non-living sources, the star's effects and the planet's conditions. Water vapour alone does not prove oceans or life.
Recurring dips and spectral shifts can reveal exoplanets. The habitable zone and atmospheric gases guide further checks for water and possible life.
Companion readings written for this learning dataset. Flashcards explain the astronomy concepts in original wording.
More to read
Which planets are easiest to find?
Imagine making a list of planets by watching stars dim. A large planet usually blocks more light than a small planet crossing the same star. It is therefore easier to notice. A planet with a short orbit also returns sooner, giving observers more chances to measure another transit.
The radial velocity method has its own selection effects. Massive planets close to their stars can produce larger changes in stellar motion. Those changes are easier to measure than very small ones.
The planets found first need not represent every planetary system. Astronomers have found giant gas planets close to their stars, called hot Jupiters, and planets orbiting pairs of stars. Super-Earths have masses above Earth's and below Neptune's. That name alone does not establish their composition or promise Earth-like conditions.
Possible water is a starting point
A planet's distance from its star helps astronomers estimate the energy it receives. Its atmosphere also affects the surface temperature. That is why a location in the habitable zone allows the possibility of surface liquid water without guaranteeing it.
Surface water is only one question. An icy world outside that zone might have water beneath its ice. Water vapour in an atmosphere does not by itself establish a surface ocean. Neither observation proves life.
Atmospheric gases need careful interpretation too. Oxygen and methane have non-living sources as well as possible living sources. Chemical reactions can remove them. Substantial amounts together may point to continuing supplies, but interpreting those supplies requires information about the star and the planet.
Checking a planet explanation
After noticing a dip, observers check the measurements and watch the star again. Similar dips at regular intervals support a transit explanation, but other causes still need testing. If successive transits come from one planet, their spacing tells us its orbital period.
Observers can seek a different measurement: shifts in the star's spectral lines. These reveal motion toward or away from us. An orbiting planet can cause that motion through gravity, even when its orbit does not carry it across the star from our viewpoint.
For some transiting planets, a little starlight passes through the atmosphere. Gases absorb particular wavelengths. Comparing measurements during and outside a transit can help identify atmospheric gases. These observations address different questions, so combining them helps test a planet explanation.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.