# Exoplanet Properties

Exoplanets (like the planets in our Solar System) have many properties which we can measure, calculate or infer. These include

Radius (r): even in our Solar System, planets range in size from Mercury at 2,440 km to Jupiter which has an equatorial radius of 71,492 km. Stellar radii can range from 0.1 solar radii up to as much as 1000 solar radii in the case of highly evolved red giant stars.

Volume (V): we can calculate a planet's volume from the equation V = 4/3 π r3. Since Jupiter's radius is approximately 30 times that of Mercury, it is greater in volume by a factor of 303 i.e. 27,000.

Mass (M): we can measure the mass of both planets and stars. We usually quote this in kg but you will often see Earth mass (5.97 x 1024 kg), Jupiter mass (1.90 x 1027 kg) or solar masses (1.99 x 1030 kg) quoted as units as they can provide an easier comparison.

Density (ρ): if we know the mass and volume of an object, we can calculate its density using ρ = M/V. This value will very often give us an indication of the planet's composition, especially if we compare it with the density of water which is 1000 kg m-3.

Albedo: a measure how much light from its parent star that a body such as a planet or asteroid either absorbs or reflects. Albedo values range from 0 (for a body that absorbs all radiation) to 1 (for a body that reflects all radiation). The average albedo is the Earth is around 0.3 but this average takes account of values ranging from charcoal (0.04) to fresh snow (0.9).

"Surface" temperature (T): we can measure the temperature of a body such as a planet or star but need to be careful when we refer to a "surface". Gaseous bodies don't really have surfaces as such so we measure the temperature of Jupiter from its uppermost cloud layer (around 140 kelvin) or the Sun from the outer layers of its atmosphere (around 5770 kelvin).

Orbital period (P): the time it takes a body such as a planet to complete one full orbit of its star. In our Solar System, this varies from 88 days in the case of Mercury up to around 248 years in the case of the dwarf planet, Pluto and even as long as Sedna's orbital period of around 12,050 years.

Eccentricity (e): all planets orbiting a star obey Kepler's First Law, meaning that they trace the path of an ellipse with the Sun at one focus. For completely circular orbits, this eccentricity has a value of 0. The Earth has a value of 0.017 meaning it hardly deviates from a circular orbit at all. Mercury is the most eccentric of the Solar System planets with a value of 0.201.

Inclination (i): most planets and asteroids in our Solar System orbit the Sun in the same direction and in a plane, known as the ecliptic. We define the Earth's inclination as 0° and as with eccentricity, Mercury has the highest inclination of our planets with a value of 7°.

Composition: related to density, we can sometimes infer the elements and compounds that go to make up a planet. In the case of Jupiter, the most abundant elements are hydrogen (nearly 90%) and helium (nearly 10%), hence their designation as gas giants. In the example of the Earth, the atmosphere is approximately 78% nitrogen, 21% oxygen however we must also consider the oceans (water and salt). When we add in the material that makes up the Earth's crust, mantle and core, we determine that the most abundant elements are iron, oxygen, silicon and magnesium.

Brightness: stars and other celestial objects usually (at least at optical wavelengths) have their brightness quoted as a magnitude. This system for brightness is a little complicated but the key thing to remember is that lower values are brighter objects than higher values (i.e. the values are inverted). Magnitudes are quoted in the optical in wavebands such as V (centred on the Sun's peak wavelength; yellow) and I (very red light; close to the infrared). Other common filters include the infrared filters, J, H and K (centred at 1.25, 1.6 and 2.2 microns respectively).

Now that you've read about some of the parameters in the activity, you can go to the activity itself.