Exoplanet Properties

There is a lot we can learn about exoplanets. How massive are they? How long do they take to orbit their star? What are they made of? Knowing as much as we can about these properties helps to work out if a planet could support life! Some of this data is measured directly. Other data is calculated or inferred using other measurements.

The list below explains some common properties of exoplanets.

Radius (r) Planets vary in size, even in our Solar System. Mercury has a radius of 2,440 km while Jupiter has a radius of 71,492 km. Planets are not perfect spheres, so the radius used is at the equator of the planet. The size of stars (stellar radii) can also vary. This is often given in terms of how much bigger than the Sun they are. For example, '2 solar radii' means twice the radius of the Sun.  

Semi-major axis The average distance from the planet to its star.

Volume (V) We can calculate a planet's volume from its radius using the formula V = 4/3 π r3. By doing so we assume the planet is spherical. 

Mass (M) If we want to, we can measure the mass of both planets and stars in kg. However, this gives us very big numbers to deal with. This is why you often see masses of exoplanets compared to the mass of the Earth, Jupiter. For example, 5 Earth masses, or 0.8 Jupiter masses.

Density (ρ) If we know the mass and volume of an object, we can use ρ = M/V to calculate its density. This value tells us what the planet is made of and what it is like inside. A good material to compare it to is water which has a density of 1000 kg m-3.

Albedo This tells us how much light a planet or asteroid will reflect. This is useful if we want to work out what its surface is made of. Albedo values range from 0 (for a surface that absorbs all light) to 1 (for a surface that reflects all light). The average albedo of the Earth is around 0.3. However, this average includes values ranging from 0.04 for charcoal to 0.9 for fresh snow. 

Surface temperature (T) This tells us how hot the outer layer is. However, we need to be careful when we refer to the surface of a planet or a star. Objects like stars or gas giants don't have surfaces. For example, we measure the temperature of Jupiter using its top layer of clouds. We measure the temperature of the Sun from the outer layers of its atmosphere.

Orbital period (P) This is the time it takes an object to complete one full orbit of its star. This is usually given in terms of Earth days or years. In our Solar System, this varies from 88 days for Mercury up to around 248 years for the dwarf planet, Pluto.

Eccentricity (e) The orbits of all planets obey Kepler's First Law. This means that their orbit is an ellipse with the Sun at one focus. The eccentricity tells us how elliptical the orbit is. For completely circular orbits, the eccentricity is 0. The Earth has a value of 0.017 which means it is very close to a circle. Out of the 8 planets in the Solar System, Mercury has the most eccentric orbit with e = 0.201.

Orbital Inclination (i) The orbital inclination tells us how tilted a planet's orbit is compared to a reference plane. For exoplanets, the reference plane is usually the line of sight from Earth.

Composition We can sometimes infer what a planet is made of using what we know about its properties. Albedo and density are particularly useful for this. 

Brightness When we talk about the brightness of objects in space, we often refer to their magnitude. This system for brightness is a bit complex but the key thing to know is that brighter objects have lower magnitudes. Magnitudes may be given for different wavelength ranges depending on which telescope filters have been used. Wavelengths you may see in this project are V (visible; the Sun's peak wavelength), I (very red light; close to the infrared), and J, H and K (infrared).

You now know which exoplanet properties can be used in the project. Next, read through the instructions and resources.