Table of Contents
What Is an Exoplanet?
An exoplanet is any planet orbiting a star other than our Sun. Since the first confirmed detection in 1992, over 5,600 exoplanets have been discovered using methods like radial velocity, transit photometry, and direct imaging. Characterizing these worlds requires calculating their physical properties from limited observational data.
By measuring a planet's mass and radius, we can determine its density, surface gravity, and escape velocity. Combined with information about the host star's luminosity and the orbital distance, we can estimate the equilibrium temperature to assess potential habitability.
Key Formulas
Where G is the gravitational constant, M and R are planet mass and radius, L is stellar luminosity in solar units, and a is orbital distance in AU.
Planet Types
| Type | Mass Range | Radius | Density |
|---|---|---|---|
| Rocky (Terrestrial) | 0.1-2 M⊕ | 0.5-1.5 R⊕ | 3-8 g/cm³ |
| Super-Earth | 2-10 M⊕ | 1.5-2.5 R⊕ | 2-6 g/cm³ |
| Mini-Neptune | 5-20 M⊕ | 2-4 R⊕ | 0.5-3 g/cm³ |
| Gas Giant | 50-5000 M⊕ | 6-15 R⊕ | 0.3-2 g/cm³ |
Frequently Asked Questions
What is the habitable zone?
The habitable zone is the range of orbital distances where liquid water could exist on a planet's surface. For a Sun-like star, this is roughly 0.95-1.67 AU. The equilibrium temperature in this zone is roughly 200-300 K, though greenhouse effects can significantly modify actual surface temperatures.
How are exoplanet masses measured?
The radial velocity method measures the gravitational wobble a planet induces in its host star. Transit timing variations can reveal masses in multi-planet systems. Direct mass measurement requires combining transit (radius) and radial velocity (mass) data.
Could we live on a super-Earth?
Potentially, but the higher surface gravity (2-3g for a 5 M⊕ rocky planet) would make movement difficult. The stronger gravity would retain a thicker atmosphere, possibly creating crushing surface pressures. Whether plate tectonics and magnetic fields exist on super-Earths remains debated.