Circumstellar Habitable Zones in Our Solar System

by Laurance Doyle - Astronomer

Jan. 08, 2001

If Venus and Mars had been switched at "birth" we might have had two habitable planets in our solar system.

In addition to Earth, the planet Venus -- if it was in Mars orbit -- might have been a nice place to live.

However, Venus in its present orbit is superhot, with about 100 times the atmosphere that Earth has, while Mars is very cold, with about one one-hundredth the atmosphere that Earth has. What happened way back then (5 billion years ago) to eventually make Earth the only planet in the solar system that could sustain water in liquid form on its surface? To find out, lets take a vastly simplified look at the birth of our star, the Sun, and the formation of its planetary system.

The Sun is likely a later-generation star. We know this because the Big Bang origin of the universe apparently could only have made the two elements -- hydrogen and helium. All the heavier elements were forged in supergiant stars that exploded as supernovae, spreading them throughout the galaxy. One of these supernova explosions just might have actually caused the initial contraction of a gas cloud that would go on to form the cocoon-like nebula where the Sun could form. For those who like to study ancestral roots, this would make that massive star our great, great, great -- add 250 million more "greats" -- grandparent.

This cocoon is called the "protosolar nebula." As it contracted (again, a very simplified version), the material that would accrete into the planets formed a disk (called the "protoplanetary disk"). Planets forming in different regions of the disk, however, should have had varying chemical compositions because areas farther from the Sun were much cooler than those close in. Thus the protoplanetary disk acted a bit like a giant smelting pot. In the outer regions, where Neptune and Uranus formed, the temperature was cool enough for ice to condense out. This is where most comets formed (comets are like giant, slightly muddy snowballs).

Closer to the Sun was the region where the higher-melting-point elements would condense out -- elements such as iron, nickel, aluminum, silicon and so on. This is the kind of material that makes up the inner planets -- Mercury, Venus, Earth and Mars. So, how did Earth get so much water if it mostly condensed out by Neptune? The key apparently lies with Neptune itself -- with lots of help from Uranus, Saturn, and especially Jupiter.

The giant planets must have formed before the Sun could completely dissipate its cocoon, as these giant planets (again, especially Jupiter) incorporated some of the cocoon's nebular material to gather their atmospheres. This is why they have so much more mass than the inner planets (Jupiter is more massive than all the other planets put together.) The giant planets (again, especially Jupiter) started to gravitationally deflect these comets out in all directions. Most (maybe more than a trillion of them) went flying out of the solar system to reside today in what is known as the "Oort Cloud" of comets (named after Jan Oort, the Dutch astronomer who first hypothesized this).

Fortunately for Earth, many comets also were directed toward the Sun as well, and some came crashing into Earth and the other inner planets. Most of these comets contained prebiotic molecules as well. These biogenic molecules formed as a result of chemical reactions that took place while the protosolar nebula cocoon was protecting them from the ultraviolet (UV) rays of the stars. (UV light "slams" into molecules with enough energy to break carbon bonds and so generally tends to keep organic molecules from forming unless they are protected in such a thick nebular cocoon.)

Thus, many scientists today think that Earths oceans are just what a few 10,000 comets would look like when they crash onto a planet that is just the right distance from the Sun. Why, you may ask, is most of our water salty? It wasnt originally. It took billions of years of rain to dissolve all the mountains of salt that used to be on land and are now part of the sea. But comets must also have rained down on, for example, Venus and Mars as well. What happened to Venus ocean? Where did the seas of Mars go?

It turns out that Venus was just a bit too close to the Sun, while Mars was just a bit too small of a planet. In the next essay we will "switch" them and see what we get! Meantime, take a moment to have a nice cool drink of comet juice and consider what a remarkable series of events brought it to you.

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Circumstellar Habitable Zones in Our Solar System