One wouldn't think that the planets Neptune and Uranus would have much to do with us. True, they are a part of our solar system, but they are incredibly far away: Uranus orbits the Sun at a distance of about 20 AU (2.8 billion kilometers, or 20 times farther away than the Earth is from the Sun); Neptune at about 30 AU (4.5 billion km). But according to Alan Boss of the Carnegie Institution of Washington, the history of these two remote planets may tell us something about the origin of life on Earth.
|According to Alan Boss, planets such as Neptune and Uranus could have formed from a dust disk that circled our Sun.
Credit: NASA / JPL
According to Boss, Neptune and Uranus could have formed from instabilities in the disk of dust and gas the circled the Sun in its earliest years. These instabilities created gravity wells, attracting more and more material. As the material collected in the center of gravity, planetary cores formed.
Boss has previously suggested that Jupiter and Saturn also formed in this manner. But Jupiter and Saturn are gas giants, and Neptune and Uranus are more like giant ice balls. Neptune and Uranus are made of the same materials as comets, with about half of their composition being water or carbon dioxide ice. Hydrogen and helium make up no more than 15 to 20 percent of each planet's mass, whereas these elements make up nearly 100 percent of Jupiter and Saturn. Why, if the planets formed in the same way, would they be so different today?
According to Boss's model, Jupiter, Saturn, Neptune and Uranus were very similar when they first formed. The answer to their present difference lies in our Sun's birth. Stars are born from Giant Molecular Clouds -dense interstellar clouds of molecular hydrogen and dust that span across light years. Depending on their size, these clouds produce dozens to several thousands of stars that are gravitationally held together in groups called 'clusters.' The largest of these clouds will produce O and B type stars - the most massive and energetic of all stars.
In Boss's model, the Sun was born in a large cloud that also produced O and B type stars. After the Sun formed, instabilities in the disk of gas and dust that surrounded the Sun led to the formation of the four outer gaseous protoplanets. Meanwhile, a nearby O or B type star formed and began to emit extreme ultraviolet irradiation (EUV). As the Sun orbited the star cluster, the early solar system was periodically exposed to this radiation. The EUV radiation was so intense it blew off the outer gas envelopes of Uranus and Neptune, leaving the planets with thin atmospheres and planetary cores of about 15 Earth masses. Eventually, either the cluster of stars disintegrated or the Sun and its planets were ejected from the cluster.
"It is not out of the question that the UV from a star could have evaporated the hydrogen from Uranus and Neptune if the star was very close for 2 million years, and if the hydrogen was in a very extended atmosphere' some Astronomical Units in size," says David Hollenbach of the NASA Ames Research Center. "It is not clear to me if it would be the EUV, or the lower energy FUV (far ultraviolet) photons that would dominate the process."
But Hal Levison of the Southwest Research Institute believes the early solar system could not have been near a star cluster - for one thing, the gravity of a cluster could have led to a very different solar system than the one we have today.
"Passing stars could have stirred things up in the outer solar system," Levison hypothesizes. "Objects in the Kuiper belt - a ring of comets located between 30 to 100 AU - couldn't be disturbed much or else they couldn't grow. This is due to accretion - the faster the objects hit, the less likely are they to stick' to each other. And once you've stirred things up it's hard to get them back to a nice quiescent orbit. Also, the violent action resulting from the stirring would grind the comets to dust, which would then be blown out of the solar system."
|Artist's rendition of Kuiper belt.
Credit: David Jewitt
Levison is skeptical that Boss's theory of Neptune and Uranus formation could work for other reasons as well.
"I'm dubious as to whether things can stay cold enough in this model," says Levison. "Water needs to act as a rock - like a solid, not a gas. Solids sink to the center, therefore they don't get blown off. This is important, because today Uranus and Neptune are half water and ices - the exact same proportion as comets. But gas always heats up when it collapses, and Uranus and Neptune would've been big balls of collapsing gas in his model."
Boss's model of Uranus Neptune formation differs from the standard model of accretion, which attributes planet core formation to collisions between particles of dust. As these particles swirled around the Sun, clumps of dust collided with more and more material, growing larger and eventually developing gravity.
Most scientists agree that the Earth formed through this slow but steady accumulation of dust and rocky matter. The other terrestrial inner planets - Mercury, Venus, and Mars - probably formed in much the same way. But problems arise when the accretion model is applied to the formation of the outer planets: there simply does not seem to be enough time for all these cores to form. Boss says the planetary cores of Uranus and Neptune would've taken more than 10 million years to form by accretion - yet the disk of gas and dust surrounding the Sun probably only lasted a few million years. In Boss's disk instability model, however, the planetary cores could have formed in only 1,000 years.
Levison, in collaboration with Ed Thommes of the University of California, Berkeley and Martin Duncan of Queen's University in Kingston, Ontario, has developed a different theory of how Uranus and Neptune may have formed. This theory says that the cores of Jupiter, Saturn, Uranus and Neptune could have all formed by accretion in the region between 4 and 10 AU. As Jupiter's and Saturn's cores gathered nebular gas, the cores of Uranus and Neptune were kicked out. What prevented Uranus and Neptune from being flung out of the solar system was the large ring of material outside the orbit of Saturn. This material acted as a braking mechanism, slowing down the planets and allowing them to achieve their present day orbits around the Sun.
"It's like a boat on a lake, the drag causes the boat to slow down," says Levison. "We've done several models where this process turns out successfully - we get a solar system much like the one we have today."
Still, Levison is the first person to say this model is far from perfect - and most likely does not pin down how the outer planets formed.
"I call it my fairy tale," says Levison. "It explains a lot of things, but I'm far from saying I believe in it. We just don't have enough information to make a final decision about the process."
Levison suggests the true story may be a combination of accretion and disk instability. As Jupiter and Saturn formed from disk instability, they would've attracted a lot of material around them. Through accretion, this material could have developed into the cores of Uranus and Neptune.
Hollenbach has a third option for how the two planets may have formed. In his model, all the outer planets were built by accretion, with Jupiter and Saturn forming first. Then either the Sun or a nearby massive star caused photoevaporation of the gaseous disk, preventing Uranus and Neptune from collecting as much gas as Jupiter and Saturn.
|Hubble Space Telescope has peered deep into Uranus' atmosphere to see clear and hazy layers created by a mixture of gases. Using infrared filters, Hubble captured detailed features of three layers of Uranus' atmosphere.
Credit: Erich Karkoschka / NASA
"In my theory, all four planets more or less form where they exist today, but Neptune and Uranus took longer to form their cores, and photoevaporation gets rid of gas much more quickly at large distances from the Sun," says Hollenbach. "By the time Neptune and Uranus formed, there was no hydrogen to be gravitationally attracted. On the other hand, Jupiter and Saturn formed more quickly in a region which maintains the hydrogen for a longer period of time."
Boss, however, doesn't think any of the outer planets could have formed through accretion.
"There are good reasons to doubt that these planets could form by core accretion," says Boss. "The time scale is just too long, unless giant planets are rare, which does not seem to be the case, given Marcy and Butler's discoveries. Other problems involve the question of actually getting accretion cores to form at Jupiter-Saturn distances or beyond-- this has not been shown to be possible, it is only assumed. Recent work on core growth suggest that it just does not lead to cores big enough to capture gas."
So where does Earth come into the picture? According to Boss, the Earth and other terrestrial planets wouldn't have been affected by the stellar radiation because they wouldn't have formed until the Sun was well away from the star cluster.
But comets would've been among the first things created in the early solar system. The ultraviolet radiation from the nearby massive star would have touched upon the comets forming in the Kuiper belt. This lengthy period of intense radiation exposure would have led to the production of unique organic molecules in the comets.
After the Earth and other terrestrial planets formed, stray comets from the Kuiper belt hit the Earth, contributing to the Earth's water supply. The organic chemistry of the comets also may have given our planet a jump-start on the origin of life.
Hollenbach, however, is skeptical about the importance of prebiotic molecules carried to Earth by comets.
"I think that objects from the asteroid belt are now considered to be the best candidate for delivering the elements carbon, oxygen, nitrogen and hydrogen to the Earth, rather than comets," says Hollenbach. Once these volatile elements were delivered to our planet, "the organic molecules can form on Earth, where there is lots of sunlight and water. You don't need the molecules to form on the comets."
"In any case," Hollenbach adds, "the UV from the early sun will also irradiate the comets, so the question becomes: do you really need the external flux? Does it produce a significant amount of molecules that wouldn't otherwise have formed? I simply don't know the answer to these questions."
Boss is computing models of disk instability to see if the disk can make the four protoplanets needed for his hypothesis to work. Hollenbach also is fine-tuning his own model of Uranus and Neptune formation. Levison is working on models that demonstrate whether the planetary cores of the outer planets could have formed through accretion.
"I think these ideas of Uranus-Neptune formation are crazy, but the standard models don't work so we have to come up with something different," says Levison. "While these models are new and exciting, most likely they're wrong. We have to start thinking of alternatives. Probably there's a method for their formation that no one has even thought of yet."