ASTR 120 ยท Lecture 12
Radial-velocity and transit surveys revealed hot Jupiters, super-Earths, and compact systems that forced revisions to formation theory.
Definition: A temporary decrease in observed stellar brightness when a planet crosses the star's visible disk.
Why it matters and connects: Transit timing, duration, and depth constrain orbital period, geometry, and planet-to-star radius ratio.
Boundary or common confusion: A brightness dip can have stellar or instrumental causes, and most planetary systems are not aligned to transit.
Definition: The line-of-sight component of a star's motion inferred from Doppler shifts in its spectrum.
Why it matters and connects: A periodic stellar velocity signal measures the gravitational reflex motion caused by an orbiting companion.
Boundary or common confusion: It yields a minimum mass unless orbital inclination is known and can be confused by stellar activity.
Definition: A systematic preference for discovering planets whose signals are larger, more frequent, or easier to separate from noise.
Why it matters and connects: It must be modeled before using detected planets to infer the true population or compare architectures with the solar system.
Boundary or common confusion: A catalog distribution is not automatically the underlying occurrence distribution.
Definition: The lower bound \(M_p\sin i\) obtained from radial velocity when orbital inclination \(i\) is unknown.
Why it matters and connects: It states precisely what the Doppler data constrain before transit, astrometry, or dynamics provide geometry.
Boundary or common confusion: The true mass can be larger, especially for nearly face-on systems.
Definition: The average number or fraction of planets in a defined radius, mass, period, and host-star range after correcting for survey completeness.
Why it matters and connects: Occurrence rates allow a fairer comparison between the solar system and exoplanet populations than raw discovery counts.
Boundary or common confusion: The value depends on bin definitions, stellar sample, detection efficiency, and assumptions about missed planets.
Definition: A change in orbital size caused by exchange of angular momentum with a gas disk, planetesimal disk, or other planets.
Why it matters and connects: Migration explains why hot Jupiters and compact systems can contain planets that likely formed elsewhere.
Boundary or common confusion: Migration is a family of mechanisms, not evidence that every planet moved by the same amount or direction.
Exoplanets test whether our solar system is typical or unusual.
Transits: planet size from fractional starlight blocked.
Radial velocity: planet mass constraints from stellar wobble.
Demographics: populations reveal architectures the solar system does not contain.
Hot Jupiters, compact multi-planet systems, and super-Earths show that planet formation and migration produce a wider range of architectures than our local example alone suggested.
The diagram links signal size to what surveys find first. Large planets close to small stars are easier to detect by transit, so raw discoveries are not an unbiased census.
A close-in giant planet is hard to reconcile with simple in-place frost-line formation. It points toward migration, scattering, or disk-driven movement after formation.
Transit data can give size, but density requires mass. A 2-Earth-radius planet could be rocky, water-rich, or gas-enveloped unless radial velocity or transit timing helps constrain mass.
A catalog dominated by easy detections does not mean the universe is dominated by those planets. Survey geometry, cadence, stellar type, and noise all filter the sample.
You discover a compact system of five sub-Neptunes inside Mercury's orbit. Which solar-system formation expectation does it challenge most: frost-line composition, long-term stability, or migration history?