ACT (Enhanced)ScienceHard
A team of astrophysicists is analyzing data from a newly discovered exoplanet. They have observed periodic dips in the star's brightness, consistent with a planetary transit, and periodic shifts in the star's radial velocity, indicating a gravitational tug from the planet. The transit data provides the planet's radius relative to the star, and the radial velocity data provides the planet's mass. By combining these two sets of observations, what fundamental property of the exoplanet can the astrophysicists calculate that neither method can determine alone?
- AIts average density.
- BIts orbital period.
- CIts surface temperature.
- DIts atmospheric composition.
Show answer & explanationAnswer & explanation
Correct answer: A. Its average density.
Density is calculated as mass divided by volume. The transit method, by providing the planet's radius, allows for the calculation of its volume (assuming it's spherical). The radial velocity method provides the planet's mass. Therefore, combining both pieces of information allows for the determination of the planet's average density.
Why the other options are wrong
- B. Both methods individually provide information about the orbital period, so this can be determined by either method, not exclusively by combining them.
- C. Surface temperature is inferred from the star's luminosity, the planet's distance, and its albedo, not directly from its mass and radius.
- D. Atmospheric composition is inferred from spectroscopic analysis of starlight passing through the planet's atmosphere during transit, not directly from radius and mass.
Exoplanet Characterization
The process of determining the physical properties of planets orbiting stars other than the Sun. Key properties include mass, radius, density, and atmospheric composition, often derived from combining different observational techniques.
- Transit method gives planet radius (from dip in star's light).
- Radial velocity (Doppler) method gives planet mass (from star's wobble).
- Combining radius and mass allows calculation of density, a key indicator of composition.
Memory trick: Transit and Wobble Reveal What's Inside