by Anthony Francis Cernosek 11/18/2003
In our quest for life-suitable worlds, we need to answer some vital questions:
What are distant planets like? (Especially those similar to Earth in mass.)
Which ones might have liquid water, and a temperature suitable for life?
How many distant worlds are truly Earthlike - with a warm, wet, sunlit surface?
Why are Venus and Mars - our nearest neighbors - so different from Earth?
How can we estimate conditions on planets we can't even see yet?
The chart at right - in development since 1996 - is based on....
Scientific fact wherever possible
A few reasonable assumptions where necessary
The principles of planetary science
A personal theory - that puts the pieces together, and offers some rough conclusions.
For more information, scroll down ...
Start simulation now !
......... Scientific facts and reasonable assumptions .......
Life is a natural process - chemistry driven by physics. Chemical processes are repeatable, given similar conditions. The visible universe is made of the same chemical elements found on Earth, and distant events are driven by the same laws of physics. (in most cases)
Biologists are convinced that all life - Earthly and alien - will be based on complex carbon molecules and will require liquid water. Both are abundant across our galaxy (and others), especially in the giant gas clouds that spawn stars and planets. Since life is a natural process of chemistry driven by physics, it can begin on any planet where the chemical and physical conditions are suitable.
Many astronomers believe that all (or most) planetary systems are built from similar interstellar material, and by a process similar (at least in the early stages) to that which formed our own Solar System. This includes the condensation of small solids inside a rotating cloud (a proto-planetary disk), thermal fractionation of the disk particles by the proto-star, and the production and scattering of billions of icy comets, which collide with all the planets and moons of the young planetary system, and supply them with water, atmospheric volatiles, and organic material.
But how many young planets will retain their water in liquid form?
........ The principles of planetary science ..........
All young planets are hotter internally (than they will be later), due to the initial heat of accretion, and due to the radioactive decay of elements supplied by nearby supernovae. (In our Solar System, short-lived Aluminum 26 and other isostopes generated enough heat to completely melt all the rocky bodies 1000 Km in diameter or larger in Earth's vicinity. ) Massive bodies are initially the hottest inside because their massive gravity produced the most violent accretion. (Earth's core is about 5000C, Jupiter's core about 20000C. )
Young planets are also hotter externally , especially those exposed to the strong radiation of their typically unstable proto-star.
Young planets have a more massive atmosphere (than they'll have later), containing more hydrogen and water. These atmospheric volatiles are supplied by comets as planets accrete, and later outgassed from the planet's interior, along with carbon dioxide and other volatiles. Massive planets orbiting far from the parent star (where comets form) end up with the most hydrogen and water.
Some of this early atmosphere is rapidly stripped away by the parent star's stellar wind, especially on low mass planets with weak gravity (and no magnetic field). The first atoms and molecules lost are those with low atomic weight - hydrogen and helium. Loss of hydrogen implies the loss of water - the only common source of hydrogen on rocky, Earth-sized planets.
As any planet evolves , it cools internally - especially low mass bodies with a larger surface to volume ratio, so outgassing slows as the solid rocky crust thickens. Planets also cool externally as their parent star "settles down". With a weaker stellar wind, stripping of atmospheric hydrogen and water also slows - especially on massive bodies with strong gravity orbiting far from the parent star.
Outgassing continues for a long time on massive bodies, gradually replenishing the atmosphere and its hydrogen and water content, augmented by more cometary impacts. If liquid water exists on the surface , exposed to atmospheric carbon dioxide and to damp surface rocks, carbon dioxide is removed into its solid form as carbonate deposits. Any carbon dioxide remaining in the atmosphere traps infrared radiation , which contributes to the global greenhouse effect, and heats the planet's surface and atmosphere.
The result- after 4 or 5 billion years - is illustrated on the chart above.
Zone 1 planets are more massive than Earth, hotter internally, and outgas strongly for a long time. Bodies in this zone received more hydrogen and water initially, and have retained more of it. The resulting planets have a denser, wetter atmosphere and a heavier cloud cover which acts like a blanket to keep the surface hot and dark. An abundance of liquid water (a deep global ocean) removes carbon dioxide from the atmosphere into solid deposits of carbonate which prevents a runaway greenhouse effect. The early Earth may have had conditions much like this.
Zone 2 planets receive less stellar radiation than Earth, and are exposed to a weaker stellar wind. Planets in this zone received a little more hydrogen and water initially, suffer less atmospheric loss, and hence have retained more of both. The result is cold planets with a thick ice cover over a global ocean. They outgas as much carbon dioxide as Earth, mostly thru undersea vents, and some of the carbon dioxide is removed as solid carbonate.
Zone 3 planets receive more stellar radiation than Earth, suffer more atmospheric loss via a stronger stellar wind, and thus lose more hydrogen and water. Outgassing of carbon dioxide is as strong as on Earth, but there is little water to act as a catalyst for its removal to solid carbonate. The result is hot, dry desert worlds with a dense, atmosphere primarily composed of carbon dioxide. Note that 500 million years from now, when our Sun swells into a red giant, Earth may become a planet much like this.
Zone 4 worlds are less massive than Earth, with a cooler interior and a thicker rocky crust, and outgassing is slow or nonexistent. With weak gravity, they lose more hydrogen and hence water, and atmospheric carbon dioxide cannot be removed. The result is cold, dry desert planets with a relatively cloudless atmosphere full of carbon dioxide.
Zone 5 worlds are truly Earthlike in mass and received radiation. These planets have cooled and lost some hydrogen and water, but still retain quite a bit even after 4 or 5 billion years. Moderate outgassing of carbon dioxide continues, but adequate surface water acts to remove this gas into solid form as carbonate. The result is planets with a warm, wet, sunlit surface and only a moderately dense atmosphere - dominated by nitrogen.
Venus , with slightly less mass than Earth, and receiving twice as much stellar radiation, is an extreme case of a Zone 3 world with virtually all of its hydrogen and water stripped away. Carbon dioxide cannot be removed from the atmosphere, yet volcanic action continues to pump more of this strong greenhouse gas out of the planet's interior. The result - an extremely dense atmosphere of carbon dioxide (90 times the density of Earth's), a runaway greenhouse effect , and a surface temperature of about 900F.
Mars , with much less mass than Earth, and receiving only half the stellar radiation, is an extreme case of a Zone 4 world. Much of its hydrogen and water has been stripped away, and the remainder frozen into polar caps and subsurface ice. Although outgassing of carbon dioxide probably ceased long ago, it is still the dominant gas in this planet's thin atmosphere since there is no liquid surface water to remove it. The result is an extremely frigid ( - 60C average), dry, and dusty desert world.
...... Author's notes .......
This chart is only a small part (about 1%) of a much larger diagram that represents ALL possible bodies (from small asteroids up to giant planets)- in ALL possible orbits ( from that of Mercury to Pluto and beyond). In other words, we predict that 1% of ALL distant planets will be roughly Earthlike in mass and received radiation. This 1% includes all 5 "Earthlike" planetary zones. Note that only 1/5th of this area (.2% of all planets) may be truly Earthlike.
Also note that the orbital distance to the parent star has been intentionally omitted from the diagram. This makes the chart useable for predicting conditions on planets orbiting any type of star - not just Sun-like (type G) stars. For instance an Earth-sized planet orbiting a cool, dim type M star could be quite warm and wet, as long as it orbits close enough to its star to receive the same amount of radiation as Earth. This is very important, since type M stars are very common, and some are known to have a planetary system. Unfortunately, few such stars have been searched for possible planets, probably because it's extremely difficult to detect planets orbiting such dim stars.
We have not yet built into this chart - or into the simulation - the possible effects of.....
* A planetary magnetic field to deflect the stellar wind and thus retain more hydrogen and water on a planet. Without a magnetic field, Earth might now be a type 3 planet - a hot, dry desert world with a large carbon dioxide atmosphere something like that of Venus. A more massive planet (zone 1 or 2) might not need a magnetic field to retain hydrogen.
A magnetic field is very helpful for Zone 3 and 4 planets that need to retain all the hydrogen and water they can.
Producing a planetary magnetic field may require....
1. A partially liquid iron core ( unlikely in low mass Zone 4 bodies)
2. A planet that rotates. This is unpredictable, and probably due to random, massive impacts like the one that created Earth's moon. Venus may have a slow rotation (and no magnetic field) because it never received a sufficiently massive impact. A planet could initially spin, but later become "tidally locked".
* Tidally locked planets - with one hemisphere permanently facing the parent star. We've assumed that all planets on our chart do rotate. Does it matter? Perhaps only on small zone 3 and 4 worlds, where the already scarce water supply might be completely evaporated from the "day" side of the planet, and re-condensed and frozen solid on the "night" side. That might render the entire planet dry and lifeless. Planets orbiting very close to type M stars might be as warm as Earth, but are likely to be tidally locked. That may be the kiss of death, at least for zone 3 and 4 worlds.
* Stripping away most of the planetary atmosphere by a massive impact (the likely origin of Earth's moon), or stripping by a nearby supernova (a likely event since young star clusters are often packed into less than a cubic light year - very close to extremely hot type O and B stars). These stripping events are particularly effective at removing water from very small bodies - (zone 4) which may already have outgassed all the internal water they had. Larger bodies like Earth (zone 5) are still outgassing water today, 4.6 billion years after it's formation. Distant large bodies might recover to somewhat wet and "life suitable" conditions after a major "atmosphere stripping" event.
* We've also ignored important events that can happen in the later evolutionary stages of star / planet formation which - judging by some of the planetary systems found recently - can be rather violent. For example.....
1. A proto-planetary disk (or PPD) tends to shrink toward it's orbital plane and expand toward ( and away from) it's parent proto-star. Some of the protoplanets embedded in a massive PPD can be dragged inward toward the proto-star and engulfed. Jupiter-sized bodies capable of raising a tide on the stellar surface may escape this fate via the gravitational acceleration of the star's tidal bulge, and end up orbiting extremely close to the parent star as "hot Jupiters". That's good for the possibility of liquid water and potential life - if the star is a cool type M dwarf, but a more luminous star would overheat the planet, vaporizing its surface water and probably sterilizing it.
The planets of the Solar System may have escaped this fate because the outer portion of our PPD was partially evaporated by a nearby supernova (there's isotopic evidence of this in some meteorites), or stripped away by the extremely strong stellar wind of a nearby massive star ( the soon-to-be supernova), or truncated by a close encounter or grazing collision with another PPD. All 3 events are probably common, since stars (and planets) always form in tight clusters with very little "elbow room". This implies that planetary systems like ours (with planets orbiting relatively far from the parent star) probably are common , although we haven't found many of them yet.
2. Gravitational interactions between 2 or more massive proto-planets can throw both into highly elliptical orbits , increasing the chance of collision between them ( and the chances for subsequent merging into an even more massive body), or ejecting one of them from the parent star. But none of these events necessarily preclude life of some kind. A highly elliptical orbit produces long summers and winters, but the average planetary temperature remains the same as in a circular orbit. Hydrogen and water would be lost as the planet approaches the star, but in this portion of the planet's orbit it is moving rapidly.
Mergers into giant planets might also produce large icy moons like Jupiter's Europa - which may have a subsurface ocean where life might exist. A sufficiently large body (Earth-sized) -with adequate internal heat and hydrothermal vents, could even be ejected into interstellar space and still have life of some kind - but not photo-synthetic life.
© 2003 by Anthony Francis Cernosek