All these worlds...
The nature of exoplanetary systems
What do we know about exoplanets?
Quantities we (sometimes) know:
●
●
●
●
●
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Mass
Radius
Semi-major axis / period
Orbital inclination
Orbital eccentricity
Multiplicity (at least a lower limit)
J
S
U
Me
V
E
Ma
N
J
S
U
Me
V
E
Ma
N
Solid or squishy?
→ A planet’s bulk density (or mean density) is a simple yet powerful tool
for determining its composition.
Assuming a spherical object, the bulk density is:
Mass of
planet
Radius of planet
Some common densities
Substance
Density (kg/m3)
Hydrogen
0.0899
Nitrogen
1.251
Water
1000
Granite
2600
Iron
7874
Ranking Task: Bulk density
Densities of the planets
Planet
Density (kg/m3)
Mercury
5427
Venus
5243
Earth
5515
Mars
3934
Jupiter
1326
Saturn
687
Uranus
1270
Neptune
1638
The degeneracy of bulk density
→ Problems which have more than one solution are known as
degenerate.
For example, what’s the composition of something with a bulk density of
3500 kg/m3?
Solution 1:
17% iron, 83% granite
Solution 2:
28% iron, 35% granite, 37% ice
Solution 3:
45% iron, 55% hydrogen
Equilibrium temperature
→ The equilibrium temperature is the temperature at the surface of a
planet under the assumption that there is no atmosphere.
Radius of the star
Equilibrium
temperature of
planet
Temperature of
the host star
Albedo
Distance to the
star
Equilibrium temperature
Temperature of the Sun:
5780 K
Radius of the Sun:
695,500 km
Earth-Sun distance:
149,600,000 km
Earth’s albedo:
0.37
Equilibrium temperature
Temperature of the Sun:
5780 K
Radius of the Sun:
695,500 km
Earth-Sun distance:
149,600,000 km
Earth’s albedo:
0.37
Equilibrium temperature of the Earth: 248 K = -13 ℉
Equilibrium temperature
Temperature of the Sun:
5780 K
Radius of the Sun:
695,500 km
Earth-Sun distance:
149,600,000 km
Earth’s albedo:
0.37
Equilibrium temperature of the Earth: 248 K = -13 ℉
Mean temperature of the EartH: 287 K = 57 ℉
Ranking Task: Equilibrium temperature
So what about those atmospheres?
→ Astronomers use
spectroscopy to
study the
atmospheres of
exoplanets.
One planet, two planets, three planets, four...
→ The number of planets in a given system is known as the multiplicity.
Kozai resonance
Summary
Important tools for understanding exoplanets:
● Bulk density
● Equilibrium temperature
● Atmospheric spectroscopy
Exoplanetary systems have extraordinary diversity!