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Explanation of Exoplanet Animations

For each planet, we’ve provided an “Exoplanet System Visualization” that shows in an animated graphic information about each planetary system. Each star has its own animation, with the star name labeled at the top. The features of these animations are explained in this annotated figure and more specific information is given below:
star color and size, planet color and size, orbital path, scale bar, time elpased.

Exoplanet System Visualization: Each star in the Exoplanet Catalog has an animation that illustrates a variety of information. The name of the star is listed at the top. In the upper-left a counter shows the amount of real time elapsed. In the upper-right is the scale bar that changes from system to system. The colors and sizes of the planets give an immediate indication of the exoplanet’s mass and size by using the color legend at the bottom. The color and size of the star vary based on the type of the star. Finally, all the planets move along in their unique orbital paths, determined from the information in the Exoplanet Catalog.

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Star Color and Size

Using the information in the exoplanet catalog, the animation shows an estimated color and size for the star.

Star Color and Size: This legend shows the meaning of the colors and sizes of the stars in the exoplanet animations. A variety of stars are shown, including the Sun, to show how the color and size scales progress. Also shown is the “spectral type” of these stars and the effective surface temperature of the star in Kelvin. See the text for more details.

In the astronomical community, stars are categorized according to “spectral type”, which is useful for distinguishing between large blue O and B stars, solar-like F, G, and K stars, and small red M stars. The legend shows a variety of spectral types and how these are represented in the animations. The color follows roughly the true colors of these stars, which is determined by the stellar temperature (except where the type of star is unknown, shown in black). While most planets are found around “main sequence” dwarf stars like our Sun, a few planets have been found around subgiant and giant stars, such as a K0III star shown (the K0 describes the temperature of the star, while the III means it is a subgiant). These are stars that are nearing the end of their lifetime, having consumed most of their nuclear fuel, and are much larger (often about 10 times larger) than their main sequence equivalents. Note that the stellar sizes are not exactly to scale compared to each other: a B0 star is about 20 times larger than the Sun and a M5 star is about 5 times as small. Furthermore, the stellar sizes are not to scale with the orbital paths: the Sun’s radius is only about 0.005 AU and would be smaller than a single pixel in most animations. Though in some animations the planet appears to go within the star, in reality each of the exoplanets in the catalog orbit at safe distances.

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Planet Color and Size

Except for a few cases, it is generally impossible to measure the actual color of the planet. The planets in our animations, therefore, are colored according to the mass of the planet.

Planet Color and Size: This legend shows the meaning of the colors and sizes of the exoplanets seen in the exoplanet animations. Some planets are shown, including Earth, Neptune, Saturn, and Jupiter to show how the color and size scales progress. Also shown is the mass of these “exoplanets” in Earth masses (1 Earth mass is 6 x 10²⁴ kg or 6.6 x 10²¹ tons) and Jupiter masses (1 Jupiter mass is 1.9 x 10²⁷ kg or 2.1 x 10²⁴ tons). For most planets, the planetary size is not possible to measure, in which case we estimate the radius. The legend shows the radius of these simulated exoplanets in units of Jupiter’s radius (71500 km or 44400 miles).

The colors are taken to be comparable to their solar system analogs: Earth-mass planets are green, Neptune-mass planets are blue, Saturn-mass planets are yellow, and Jupiter-mass planets are brown. Planets much smaller than Earth are more gray, while planets much larger than Jupiter are more red. While these colors are arbitrary and not representative of their true colors, this color scheme helps to quickly illustrate the mass of the planet. Planets are shown in black if their mass is unknown or cannot be estimated.

At the bottom of every animation, there is a color bar to remind you of this scaling, with the positions of Earth, Neptune, Saturn, and Jupiter shown. Note that most planets are discovered with the radial velocity technique, which only determines the “minimum mass” of the planet. The animation illustrates the minimum mass of the planet (as listed in italics in the catalog) if the true mass is not known.

At the bottom of each animation, this “mini-legend” helps decipher the mass of the animated exoplanets.

The size of the planets is also meant to be representative of their actual sizes. Unless the planet transits its parent star, the size of the planet cannot be measured and the actual sizes are estimated based on assumed densities. Like the star sizes, the planet sizes are not to scale to each other (Earth is about 10 times smaller than Jupiter) or the the size of the orbits (1 Jupiter radius is only 0.0005 AU!).

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Orbital Path

The orbital path of each planet is drawn as a thin black line. This is what the planetary system would look like if viewed from above the orbital plane. (Note that the animations are not showing the planetary system as seen from Earth.) Johannes Kepler in the early 1600’s showed that the orbits of the planets are ellipses (Kepler’s First Law). The eccentricity of an orbit describes how non-circular or stretched the orbital path is. An eccentricity of 0 corresponds to a purely circular orbit. For low eccentricities (less than about 0.1), it is hard to measure the small deviations from circularity and such planets are often assumed to be on circular orbits. In the animations, small non-zero eccentricities can be seen as an apparent asymmetry in the placement of the orbit. Some planets, such as HD 80606 b, have very large eccentricities, often due to perturbations from another planet or star. When the orbit is clearly non-circular, it is also easy to see another effect of eccentric orbits: the planet moves at a changing speed. Kepler’s Second Law found that planetary motion sweeps out “equal areas in equal times”, meaning that planets move faster when they are closer to their central star.

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Scale Bar

Every ellipse has a “semi-major axis”, which is defined as half of longest distance across the ellipse. In circular orbits (when the eccentricity is 0), the semi-major axis is just the radius of the circle. We have used the semi-major axis of the most distant planet as the scale bar for our animations. This scale bar is shown in the upper right-hand corner, i.e. “0.8 AU” means that the semi-major axis of the planet with the largest orbit is 0.8 AU, and the blue line is 0.8 AU long in this animation. The length of 1 Astronomical Unit is defined as the semi-major axis of the Earth’s orbit around the Sun (about 150 million km or nearly 93 million miles). Browsing through the catalog, you will see that planets have been found at a variety of semi-major axes. Some “hot Jupiters” have semi-major axes of only 0.02 AU (i.e. 50 times smaller than Earth’s orbit, about 3 million km or a mere 1.9 million miles), while other planets have semi-major axes like that of Jupiter in our own solar system, which orbits at 5.2 AU (778 million kilometers or 483 million miles). This vast variety in scale means that it would be difficult to clearly show the orbital paths of all the known exoplanets if they all were plotted at the same scale. Therefore, each stellar system has its own scale bar and intercomparisons between the actual sizes of each system must account for the fact that the scale bar changes from system to system.

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Time Elapsed

The same variety in size scales is also seen as a variety in time scales, with some planets completing full revolutions in less than a day (!) and other planets with estimated orbital periods of hundreds of years. Each animation is run at a different speed and the total time elapsed is shown in the upper left-hand corner as the animation proceeds. This helps to show the length of the animation in real time. Note that the animations are looped and, for most planets, the planet appears to be going around continuously. In this case, the total animation time refers to the orbital period of the outermost planet. For some systems (e.g. 55 Cancri), the animations cover only a small fraction of the orbital period of the outermost planet. In these systems, the timescale of the animation is chosen so that the motion of the innermost planet remains smooth and the total time of the animation does not correspond to the orbital periods of any of the planets. This will also cause the outer planets in these systems to artificially “jump” to another part of the orbit when the animaiton loops.

The Exoplanet System Visualization animations are free for non-commercial use. If you use these animations, please give credit to The Planetary Society.

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