Planets of Other Stars

Promise of Life in the Universe


Since 1995 an amazing range of planetary sized masses have been detected around other stars using indirect methods. A natural assumption is that these masses are actual planets, but they are planets like nothing astrophysicists expected - Jupiter-like planets in orbits closer to their stars than anyone imagined possible.

Exosolar Planet Search

The shadow of one such planet has been detected, and can be observed even by amateur astronomers, so it seems likely that the masses are the bizarre planets they seem to be. Theorists have modified their models of how planets can form, finding ways in which star-systems with multiple Jupiter-sized planets can interact catastrophically, sending planets crashing into their star or careening off into the void.

In between these extreme fates are the close-in "Hot Jupiters" that were amongst the first and most surprising planets discovered. These orbit their stars so close that their "years" are less than a week long, and they are so hot that "rock vapour" forms a major atmospheric component. From their night sides such worlds would glow red or yellow with the heat.

Extrasolar Visions

Other planets have been discovered in cooler orbits, some cool enough for liquid water to accumulate on the right planetary surface. Jupiter-like planets would not be able to sustain oceans, but like Jupiter such planets will have systems of moons, and these might provide suitable locations. At present only very large planets are detectable by astronomers as a result of the limitations of the methods used - only planets close to their stars orbit fast enough and perturb their stars strong enough to be noticed. As detectors are refined and observation periods lengthened more normal and even smaller planets are being discovered.

But let's imagine the moons of a Jovian planet for now. Firstly how big does a moon have to be to support Earth-like conditions? Our solar system provides a clue - the planet Mars. Currently Mars is geologically inactive compared to the Earth, unable to sustain the geochemical cycles that maintain our Earth's water-oceans and light atmosphere. Long ago Mars lost its magnetic field and the solar wind stripped away most of its atmosphere, which was further thinned out by impacts of asteroids. The rest was soaked up by the Martian soil. If Mars was perhaps twice as massive it would probably be habitable today - though rather cold.

This means a moon with roughly 1/5 Earth's mass would be habitable. Since a planet's density, mass and radius are related [higher mass causes a higher central density thanks to gravitational compression], a planet with 1/5 Earth mass and roughly Earth-like composition would be 8,000 km across and have a gravity of one-half Earth's.

table 1 - planets compared

Planet Diameter (km) Density (grams/cm^3) Mass (Earth = 1.0) Gravity (Earth = 1.0)
Moon 3,476 3.34 0.0123 0.165
Mars 6,790 3.93 0.1078 0.378
Habitable Moon 8,000 4.5 0.2 0.5
Earth 12,756 5.515 1.0 1.0

That would be a big moon, over 16 times the mass of our Moon. But would the primary planet need to be 16 times bigger than Jupiter? That's a tricky issue and best explored by looking at analogies from our Solar System. Jupiter, Saturn and Uranus have systems of 'regular satellites' which have orbits with a geometric series relationship between them - leading some to believe that they formed by an orderly process associated with the planet's own formation. Jupiter's four Galilean satellites are the most obvious of these - Io, Europa, Ganymede and Callisto. The Galileans seem to get progressively wetter the further out from Jupiter they orbit. Io seems to be dry like the planet Venus, and has intense sulphur vulcanism covering its surface. Europa is covered in a 100 kilometre layer of ice/water, which seems to have formed from water out-gassed from its rocks - it formed hot enough not to accumulate primordial ices. Ganymede and Callisto seem to be half-rock and half-ice - imagine Europa and Io wrapped in ice of a water/ammonia/carbon dioxide mixture. In all the rock component's mass is about that of our Moon.

The regular satellites of Saturn and Uranus, since they formed colder, seem to be like Ganymede and Callisto, though much smaller, weighing in at about 1/25 th of the mass of those moons. Saturn's Titan is comparable in size to Ganymede, but is not a regular satellite and might be a captured planet once independent of Saturn. So the mass relationship between moons and planets is non-linear - Jupiter has bigger moons because it is bigger than Saturn and Uranus, but while Saturn outweighs Uranus 6-to-1 they have similar sized regular moons. Perhaps then a Jovian planet doesn't need to be 16 times bigger than Jupiter to have habitable moons?

Another factor affecting habitability is climate. Many of the Jovians in roughly Earth-like orbits actually vary their distance from their stars quite dramatically. On Earth our seasons are driven chiefly by how much sunlight reaches a particular patch of ground over the course of a day - in Winter days are shorter and the ground is warmed proportionately less. A Jupiter-like planet would suppress seasons like this on its moons by minimising tilting of their rotational axis, but if the planet's orbit swings between the distance of Venus to the distance of Mars from the Sun over a year, the temperature variations could be quite dramatic!

A recent article in Discover magazine reports on climate simulations of planets with highly elliptical orbits. With an eccentricity of 0.3 the orbit varies from Venus' orbit [0.7 AU] to half-way to Mars [1.3 AU], but the weather is relatively stable. A similar climate occurs for an eccentricity of 0.4 - ranging from 0.6 AU to 1.4 AU. Even the extreme eccentricity of 0.7 allows habitable climates near the Poles.


Oceans and a dense atmosphere moderate these variations, but immense winds, storms and rain-fall could be typical. Tidal interactions between the moons of a Jovian would have significant effects too, perhaps causing regular earthquakes and volcanic eruptions like nothing on Earth.

Such worlds could be homes to multiple intelligent species - imagine four Jovian satellites, all habitable. Once life began it would easily spread between the moons via impacts throwing debris and cells into space. So life in such a system would be related, but evolving and adapting to quite different conditions - length of day, tides, vulcanism, water content and so forth would vary between the moons. Perhaps the planets would grade between various stages of development - the innermost might lag due to stronger vulcanism, the outermost might age quicker due to lower vulcanism from weaker inter-moon tides. The inner then would be like Earth in the Pre-Cambrian, while the outer would be Earth after geochemical cycles grind to a halt and quasi-Venus like conditions evolve. Plenty can be imagined and much would be plausible.

Currently several large Jovians are known in near-circular Earth-like orbits. If they have regular satellite systems then I suspect they will have habitable moons in abundance. Perhaps they will have oxygen-rich atmospheres, but they might be like Earth prior to the dominance of oxygen-nitrogen in the atmosphere - carbon-dioxide rich worlds, perhaps shrouded by photochemical smog from methane break-down in the upper atmosphere? Or perhaps they will be volcanically quiet, dry and wrapped in sulphur-rich clouds like Venus? Presently no one can say, but anything is possible. Question is: do you want to find out?

So how can we get to other star systems?

And how can we do it in a reasonable time?

Planets of Other Stars II: Getting There

Planets of Other Stars III: Getting There Quickly

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