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DataThe 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.
ScaleThe 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.
PlanetsMassThe 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.
Note that often this is a measure of minimum mass. In
reality the planet could be more massive.
Semi-Major AxisThe location of a planet along the graph represents its semi-major axis -- a measure of its distance from
its star.
Orbital EccentricityThe 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 DwarfsPlanetary 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 SystemsRadiusThe 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.
TemperatureThe 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 ZoneIf 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 LockingWhen 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:
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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Copyright © 2007-2022 Mike Moyer |
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