Question

in the search for a \goldilocks planet,\ which of the following describes how spectroscopy is used to identify potential earth - like planets?
by measuring the temperature of the star around which the planet orbits
by determining the planets distance from its star to find its orbital period
by calculating the planets mass and density to see if it has a solid surface
by measuring the spectral fingerprint of a planets atmosphere to detect elements and compounds that may indicate habitable conditions

Explanation:

Spectroscopy involves analyzing light. The spectral fingerprint (spectrum) of a planet's atmosphere can show which elements and compounds are present. For example, the presence of water vapor ($H_2O$), oxygen ($O_2$), or carbon dioxide ($CO_2$) can give clues about habitable conditions. Measuring the star's temperature (first option) is not spectroscopy's main use for identifying Earth - like planets in this context. Determining orbital period (second option) is more related to orbital mechanics (e.g., using Kepler's laws) rather than spectroscopy. Calculating mass and density (third option) is often done through gravitational effects (e.g., transit - timing variations or radial velocity methods) not spectroscopy.

Answer:

By measuring the spectral fingerprint of a planet's atmosphere to detect elements and compounds that may indicate habitable conditions.