Hot Jupiters and Rare Earths: Planets are common. Are we?

Are Earths Rare?Until a few years ago, planets and solar systems other than our own were the stuff of theory and informed speculation. Before that, belief in this notion of other worlds (or lack thereof) was fodder for myth and religious dogma. As our knowledge of the universe moved from the realm of guess work to hard data, we came to see our sun as one of countless stars, and not very remarkable at that.

We also came to expect that our solar system was probably unremarkable and that others would be variations on a common theme. When the first planets outside our solar system (extrasolar planets) were located 5 years ago, they showed us that reality, if anything, is not what we expected to find.

So far we only know about the extrasolar planets we have been able to find. To date 35 have been identified. While the tools are impressive in their capabilities, they are still only able to detect the easiest to find (i.e. largest) planets. A recent announcement by veteran planet hunters Marcy and Butler has shown that refinements can and will be made to these detection methods allowing smaller planets to be discovered.

While Earth-sized planets have yet to be discovered, nothing suggests that they will not be found - eventually. At some point Earth-based detection methods will run up against the laws of physics and space-based planet detectors will assume the task of searching for ever smaller worlds. The tools to find these "pale blue dots" as Carl Sagan called them, are now in the planning stage and should be operational within the next decade or so.

At the First Astrobiology Science Conference, held in April 2000, Alan Boss from the Carnegie Institute of Washington addressed the issue of the large planets that have already been found. These planets are larger than Jupiter and orbit very, very, very close to their parent stars - often closer than Mercury orbits our own sun, hence their nickname: "hot Jupiters".

Only with the recent announcement by Marcy and Butler of two new Saturn-sized planets, has anything smaller been found. Still, they too orbit very close, thus making their detection much easier than small rocky worlds at Earth-like distances.

Using our own solar system as a model of what we thought to be 'average', astronomers had expected to find large gas giants. They had expected that these large planets would be far away from their sun, that they'd be in circular, low eccentricity orbits.

Solar systems form from a solar nebula, a swirling collection of dust composed of a variety of materials. Over time this nebula contracts and becomes a more regularly shaped disk. Small irregularities develop in this disk that eventually form small bodies or planetesimals. As time progresses, these planetesimals accumulate material, eventually giving rise to planets.

According to Boss, giant gas planets form in 8 or so million years. Classic theory suggests that a small rocky core forms first from planetesimals. As it reaches as size of 10 Earth masses it begins to attract lighter elements.

In order for these large planets to have enough material to form they need to form far enough from their parent star so as to be outside the so called "water line". At this distance stellar heating is low enough that water and other ices are stable providing ample materials for large planets to form. Closer in, heating of the solar nebula is high enough that these volatile materials cannot remain in solid form leaving only rocky and metallic materials solid and available for planet formation.

When extrasolar planets were first detected, astronomers were somewhat perplexed to see large as giant planets close to their home star - somewhere that existing theories suggested they had no right to be. Again, our solar system has its gas planets in the outer regions - where they should be according to theory. According to Boss, these unexpected gas planet locations sent astronomers of to ponder their preconceptions. In asking why these hot Jupiters were so close to stars it soon became obvious: they move. They move in one (or both) of two ways. One process involves a game of gravitational billiards.

According to one model, giant gas planets may push each other around. If, for example, you have 3 large gas planets forming in a solar system out in regions where ices are abundant, they will excite each other's orbital eccentricities. Over time this will cause orbits to shift around. Eventually planets will actually swap their relative order with respect to one another and their parent star. As this happens, the inner most planet moves outward, the outermost planet moves inward, and the planet in the middle is ejected from the solar system altogether While this model tends to explain why many of the hot Jupiters discovered thus far have high orbital eccentricities, this is not the leading explanation.

The other, more accepted model involves interactions between forming planets and with the dust disk from which they are forming. An analog to this process has been observed in Saturn's giant ring system. Small saturnian moons orbiting close to the rings tend to disrupt the organization of the rings. In so doing, spiral density waves are produced. These perturbations result in movement of the moons themselves via gravitational interactions. On the larger scale of a solar nebula, so the theory goes, spiral density waves clear out a space within the developing dust disk and locks the planet into a position in the cleared space. Later, as nebula moves inward and material falls into the sun, the planet moves inward, hence closer to the local star.

As is always the case with the universe one thing always affects another. The closer a large Jupiter-sized planet is to its local star during planetary formation, the smaller the chance that Earth-like planets will form. Evidence of this effect can be seen in our own solar system in the asteroid belt between Mars and Jupiter where Jupiter's gravitational influence has either hindered planetary formation or caused objects to collide and fragment before they achieved planetary size. As such, if the object is to find more potential Earths, then the emphasis should be to search within solar systems wherein large gas giants form further out than those currently known.

Following Alan Boss was Peter Ward from the University of Washington. Ward caused a bit of a media stir recently when his provocative book "Rare Earth" hit the bookstores. The popular perception of the ubiquity of intelligent, complex life in the universe is pervasive and is a mixture of ideas put forth by the likes of Carl Sagan and Frank Drake and the somewhat less factual versions put out by Hollywood.

Ward and his co-author David Brownlee challenge this assumption head on. They contend that while simple microbial life is probably rather common throughout the universe, more complex life forms (including humans) are rather are by comparison. In laying out their story, Ward and Brownlee list a myriad of things that have to go just right in order for complex (metazoan) life to arise - and thrive while large impact events, and a host of other phenomena seek to wipe it out. Meanwhile, having Jupiter in just the right orbit, a large moon, plate tectonics, and the right starting materials are also required. The chance that all of these factors occur in a fortunistic fashion as they have for us on Earth is rather unlikely according to Ward and Brownlee.

According to Ward, the fossil record shows that bacterial life forms easily and rather early in a planet's history (or at least it did on Earth). On Earth more complex forms took another billion years to form. Ward suggests that more complex life forms were "waiting" for a more suitable environment to form.

Snowball EarthThe odds facing complex life and ecosystems on Earth are rather stiff - indeed life has almost been snuffed out many times. There have been ten or so mass extinctions starting with the early bombardment experienced across the inner solar system. Each dealt a strong blow to the ecological balance on Earth. Most notably are the so called "Snowball Earth" episodes where the entire surface of Earth was covered with ice.

While these events certainly took their toll on Earth life, some feel that they may not be bad things in the overall scheme of things. Some scientists think that these cathartic events leads to less diversity among life forms, while others people think that it increases diversity by opening chances for evolutionary innovation. Still other think that these periodic mass extinctions have no impact at all. Ward said that he was of the opinion that these impacts reduce biological diversity over time.

According to Ward, "once a planet has been infected by bacteria, mass impacts cannot sterilize it, whereas animal life is easier to extinguish." In the past decade vibrant ecosystems have been found miles under the sea and miles within the Earth's crust - places where all but the most colossal impacts would leave more or less intact.

Asteroid and comet impacts aren't the only natural phenomena that can wipe out surface life. Nearby supernova explosions, and gamma ray bursts sources can also provide a most devastating blow. On Earth plate tectonics and other climatological factors provide for a stable climate such that complex life can emerge. Anything that serves to make a planet less habitable for long periods would work against the development of complex life forms.

As mentioned by Alan Boss, Jupiter's presence within our solar system is a blessing whereas having a Jupiter closer to Earth would be a curse. Jupiter's location in our solar system serves to deflect comets falling inward from the outer solar system from reaching the inner solar system in great numbers. According to Ward this and reduces impact risk to Earth by factor of 10,000.

Is complex life common?Not only has our solar system's construction favored the evolution and perpetuation of complex life, but our place within our galaxy may also be fortuitous. Ward posed the question as to whether there are regions in our galaxy where impact rate is higher or lower that we experience. As you get closer to the center of the galaxy, stars are closer - hence their Oort clouds are closer and the chances for interaction between them are greater. As Oört cloud interactions increase so would the number of comets thrown inwards thus increasing the risk impact.

A few years ago we expected to find other solar systems like our own but instead have found weird ones that fly in the face of what we thought we knew. 20 years ago we thought we understood the extent to which life had colonized the Earth and what should be possible elsewhere. Now we find life that can live in battery acid and inside nuclear reactors and deep within the earth's sea and crust - places where they have no right to be, if you read the old text books.

If our search for life and worlds elsewhere has taught us anything thus far, it is that our world is both rare and common. Moreover, it has shown us that Earth and its immediate neighborhood can be a good guide to what is possible - but it is, after all, only one example among trillions.

SpaceRef Resources

Extrasolar Planets, Astrobiology Web
Extrasolar Planets, SpaceRef Directory
You've heard of global warming - How about global freezing?, SpaceRef
Of Planetary Transits Near and Far, SpaceRef

Background Information (Including Astrobiology Science Conference abstracts)

Gas Giant Planet Formation and The Implications for Habitable Planets, [Oral Presentation Abstract] Alan Boss, Carnegie Institution of Washington
Numerical Simulations of Triggered Star Formation, [Poster Abstract] Harri A. T. Vanhala and Alan P. Boss, Carnegie Institution of Washington
Factors leading to the appearance and survival of metazoan equivalents on habitable planets [Poster Abstract]
Spin Axis Stability of Tidally Evolved Planet-Satellite Systems [Poster Abstract]
Snowball Earth and the Origin of Metazoa, [Poster Presentation] Paul F. Hoffman, Harvard University
Snowball Fight, Nature
Snowball Earth episode 2.4 billion years ago was hard on life, but good for modern industrial economy, research show, press release, Cal Tech
Paleoproterozoic snowball Earth: Extreme climatic and geochemical global change and its biological consequences, Proceedings of the National Academy of Science [Abstract - subscription fee required for full access]
Snowball Earth, Scientific American, January 2000
Earth's Oceans Destined to Leave in Billion Years , press release, Penn State
Planet Hunters on Trail of Worlds Smaller Than Saturn, NASA press release
NASA's Ames Research Center Uses Transit Photometry to Confirm Existence of Extrasolar Planet Circling HD 209548, NASA press release

Books by the individuals mentioned

"Rare Earth", by Peter Ward and Alan Brownlee (Amazon.com)
"Looking for Earths: The Race to Find New Solar Systems", by Alan Boss (Amazon.com)


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