Graphical Comparison of Extrasolar Planets

About Me

My name is Mike Moyer. I'm a software developer by trade, but I have always had a strong passion for astronomy. In the last few years, I've been avidly following the discoveries of extrasolar planets. I finally decided to try my hand at a tool to visualize and compare these other solar systems with our own.
This website was designed with the goal of giving casual astronomy buffs a quick overview of the planetary systems of other stars. I have strived to represent the data accurately, and to fully explain how the graphs are rendered, but this site should not be confused with a scientific resource. Where possible, I have included links to other resources where the planetary data is more rigorously presented.

About The Graphs

Data

The data for this webpage is pulled periodically from The Extrasolar Planets Encylopaedia, run by Jean Schneider. Ultimately, I would like to supplement this with data from other sources.

Scale

The graph for each system is plotted with the central star on the left. Distance from the star increases logarithmically towards the right. The scale for every system shown together on the page is the same, allowing direct comparisons. However, this scale may change based on which set of systems is being displayed.

Planets

Mass

The size of a planet on the graph indicates its mass. Specifically, the width scales as the cube root of the mass. If planet X is eight times as massive as planet Y, it will appear twice as wide on the graph. Earth was arbitrarily set to be five pixels wide.
0.5 MJ 1 MJ 2 MJ
Note that often this is a measure of minimum mass. In reality the planet could be more massive.

Semi-Major Axis

The location of a planet along the graph represents its semi-major axis -- a measure of its distance from its star.

Orbital Eccentricity

The eccentricity of a planet's orbit is indicated by the bracketed line above the planet. As the planet sweeps out an ellipse around the star, it gets as close as the inner bracket, and as far away as the outer bracket. If the orbital eccentricity of a particular planet is not known, no bar is displayed.

Brown Dwarfs

Planetary bodies that are large enough can exhibit nuclear fusion in their core, at least briefly. These objects are known as brown dwarfs. A commonly cited lower mass limit for brown dwarfs is thirteen times Jupiter's mass. Planets this massive are shown with a different icon on the graph.

Stars and Systems

Radius

The central star is displayed at the left of the graph. Its size corresponds to the main scale. If the radius of a star is unknown, a question mark is displayed at its edge.

Temperature

The star's color indicates its temperature. Red stars are relatively cool, yellow stars like the Sun are hotter, and white and blue stars are hotter still. If the temperature is unknown, the star will be gray.

Habitable Zone

If a star's luminosity is known, we can estimate how much radiation its planets receive. Planets that receive moderate amounts of radiation (similar to Earth) are said to be in the "habitable zone" where liquid water--and possibly life--can exist. The habitable zone is represented by a green line in the graph. The size of the habitbable zone, even in our own solar system, is not exactly known. For the purposes of this website, I've assumed the Sun's habitable zone to be between the orbits of Venus and Mars. This is a very rough estimate.
Note that big, hot stars have much wider habitable zones than smaller, cooler stars, but because the graph's scale is logarithmic, all the green bars appear to be the same size.

Tidal Locking

When two bodies orbit each other closely, it is possible for one or both to become tidally locked. That is, the body's orbit and rotation become synchronized due to gravitational forces. Our own moon is tidally locked to the Earth, and as a result we only ever see one side. Mercury is tidally locked to the sun, and this manifests as a 3:2 spin-orbit resonance.
Tidal locking is indicated in the graph by the shaded arrows. Planets to the left of the arrows are likely to be locked. Planets to their right are unlikely to be locked. Planets within the arrows are less easy to predict.
A tidally locked planet is a poor candidate for life, even if it is in the habitable zone. One face of the planet will be baked, and the other half will be frozen in eternal night.
Wikipedia offers some equations for estimating how long it will take for a satellite to become tidally locked to its primary. Alternately, if you know the age of a system, you can guess whether a given planet will be tidally locked. Using all of Wikipedia's suggested approximations, substituting values, and solving for a yields the equation:

Which "simplifies" to:

Where...

  • a is the distance from the primary at which the satellite will be locked
  • t is the age of the system
  • G is the gravitational constant
  • p is the mass of the primary
  • s is the mass of the satellite
  • R is the radius of the satellite
  • mu is the rigidity of the satellite
  • Q is the dissipation function of the satellite
  • w is the initial spin rate of the satellite
  • Calculating whether each individual planet is likely to be tidally locked is too difficult. First, we do not possess all the necessary inputs for most of the currently known planets. Second, on a strictly technical level, the numbers involved are literally astronomical. The calculations involve such things as the age of the solar system in seconds, the mass of the sun in kilograms, and the volume of Jupiter in cubic meters. PHP fails quite spectacularly when handling these values.
    Instead I used Excel to calculate two extreme cases for the Sun: a tiny planet (Mercury) and a large planet (Jupiter), each initially rotating once every 10 hours. The former yields a distance of 0.472 A.U., and the latter 0.844 A.U. From this we can calculate those values for any star, as long as we know the mass (m) and age (t) of the star:
    If either the mass or the age of the star is unknown, the tidal locking arrows will not be displayed.


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