The Search for a Planet Suitable for Life
There are many theories about solar system formation.
In one, a passing star pulls a cigar-shaped filament out of the Sun, and the filament later breaks up and forms planets. Analysts abandoned this idea some time ago. No one could produce a computer-based scenario that didn’t end up with most of the extracted material falling back into one of the two stars. Two stars passing close to one another without colliding have too much angular momentum to fall into orbit around each other. There is no third body to carry away part of the angular momentum.
The theory was unpopular anyway because it makes planetary system formation a rare accident. Lots of people want plenty of planets to nourish their hope of someday finding extraterrestrial intelligence.
The stellar flyby with tidal forces pulling out material also fails to explain the diversity of the planets in our solar system. Our giant gas planets, Jupiter, Saturn, Uranus, and Neptune are rich in hydrogen like the Sun. Yet they are separated from the Sun by four rocky planets, Mercury, Venus, Earth, and Mars, plus the asteroid belt. These inner planets are rich in iron and other heavy elements of the kind needed for life. And then we have Pluto, Sedna, and the Kuiper Belt of asteroids beyond Neptune. Astronomers think they are similar to the Earth-like planets. We have the Sun and the giant planets made of one kind of material, with an inner and outer band of planets made of another kind of material.
Another idea is that the Sun gradually captured different kinds of planets. That would explain their diversity, but not why they formed groups. Why did four of the largest rocky planets go to the inside close to the Sun, when the rest of the rocky planets stayed outside the ring of the four giant gas planets? Once the planets are captured, initially they are likely to have many highly elongated, overlapping elliptical orbits. Getting them all to settle down into nearly circular, nicely concentric, well-nested non-interfering orbits like those in our solar system is not easy.
The theory of planetary capture also founders on the problem of angular momentum. If the Sun captures a passing planet it must somehow dissipate the planet’s kinetic energy and angular momentum so the planet will fall into orbit around the Sun. The only way to dissipate the energy is to have the encounter in a star-forming, dusty region with multiple encounters and transfers of energy to other planets or stars that aren’t captured.
This is the basis of the accretion disk theory, the most popular theory now. Recent news articles have described work on accretion theories.[i] Modelers think that planets coalesce from a swirling disk of gas and dust around new stars. Yet they are hard put to explain why light-weight Sun-like elements coalesce in a middle ring between inner and outer rings of rocky elements. The middle ring has elements like those of the center. Unlike elements might go to the outside, but why would a good portion of them seek an intermediate place between the center and the middle ring?
In the solar system, the accretion disk theory only begins to work when the dust gathers into small lumps called planetesimals. In our system Jupiter has most of the angular momentum. As soon as modelers get the parameters right to explain the angular momentum of the inner planets out to Jupiter, they get stuck with too-long formation times for the outer planets.
Science devoted an issue to planetary systems a few years ago. People without scientific training can still generally understand the news articles and reviews that accompany and interpret the scientific reports.[ii] The news editors on page 65 refer twice to “chance.” “Focusing on the four inner or terrestrial planets of our system, Richard A. Kerr (p. 68) finds that modeling studies have given researchers a new respect for the role of chance in determining these planets’ structure and composition, and therefore their ability to support life.” Jupiter has four large moons that Galileo originally discovered with a small telescope. Some researchers take these moons as a model in miniature of a planetary system. “In striking parallel with the larger solar system, the Jovian system may have formed by chance, each world is unique, and Europa may harbor life.” (We will have many questions about life under Europa’s ice when we discuss the thermodynamics of life in a later chapter.)
Physics Today also devoted an issue to planetary diversity.[iii] There is special interest in extrasolar planets. The Sun is not the only star with planets. Astronomers have discovered more than 100 planets orbiting other stars. Methods reported in April 2004 could only discover giant planets, of the size of Jupiter, or Saturn, but not as small as Neptune. Jupiter’s mass equals 318 Earth masses. Saturn’s mass equals 95 Earth masses, or 0.30 of Jupiter’s mass. Neptune’s mass equals 17 Earth masses, or 0.054 of Jupiter’s mass. Reported extra-solar planets have from 0.11 to 17.5 Jupiter masses. Until recently the discovery of extrasolar planets gave great hope to those who seek other places in the universe where life like ours may flourish. Now the hope has waned.
Especially disappointing has been the discovery that many extrasolar planets come very close to their parent star. That makes them too hot for hydrocarbon-based life. Others are “wild,” with highly elliptical, eccentric orbits. They spend long periods of time far from their parent star in the dark and cold, but periodically rush in close to fry any precursor of life that may have formed. Our giant planets, Jupiter, Saturn, Uranus, and Neptune, are tame. They move sedately in nearly circular orbits that nest nicely within one another. This unusual arrangement protects the Earth and the other inner planets.
Less than one in a hundred of the extrasolar planetary systems discovered so far have their Jupiter thoroughly domesticated. The one known example is our own solar system. This fact has raised the question: Is the process that made our solar system very unusual? Is it markedly dissimilar to the process that made all the known extrasolar systems?
When astronomers first discovered extrasolar planets many scientists thought they were getting statistics on the occurrence of Earth-like planets. The methods that detected the first hundred extrasolar planets couldn’t detect a planet the size of Earth. But what if the Jupiter-like extrasolar planets were representative of extrasolar systems similar to ours? Now some astronomers have examined that assumption.[iv] According to their article there may be two mechanisms for planet formation. The mechanism most studied applies to our solar system. But there may be another mechanism that only forms giant gas planets. It appears that the second mechanism operates much more frequently, because the extrasolar planets already found have very different orbital characteristics from those of our Jupiter. They may not be good places to look for life at all. In that case we still have no statistics for the probabilities of finding a solar system like ours. One can’t estimate the average distance between systems similar to ours if we know of only one system. The occurrence of life may still be a miracle.
[i] Ahrens, Thomas J., “The Origin of the Earth,” Physics Today, 47 (Number 8‑Part 1, August 1994), pp. 38–45.
[ii] Kerr, Richard A., “Making new worlds with a throw of the dice,” Science, 286 (Number 5437, 1 October 1999), pp. 68–69.
[iii] Physics Today, 57 (Number 4, April 2004), pp. 43–83.
[iv] Beer, M. E., A.R. King, M. Livio and J. E. Pringle, “How special is the Solar System?” Monthly Notices of the Royal Astronomical Society, 000 (30 July 2004), pp. 1–6.
In one, a passing star pulls a cigar-shaped filament out of the Sun, and the filament later breaks up and forms planets. Analysts abandoned this idea some time ago. No one could produce a computer-based scenario that didn’t end up with most of the extracted material falling back into one of the two stars. Two stars passing close to one another without colliding have too much angular momentum to fall into orbit around each other. There is no third body to carry away part of the angular momentum.
The theory was unpopular anyway because it makes planetary system formation a rare accident. Lots of people want plenty of planets to nourish their hope of someday finding extraterrestrial intelligence.
The stellar flyby with tidal forces pulling out material also fails to explain the diversity of the planets in our solar system. Our giant gas planets, Jupiter, Saturn, Uranus, and Neptune are rich in hydrogen like the Sun. Yet they are separated from the Sun by four rocky planets, Mercury, Venus, Earth, and Mars, plus the asteroid belt. These inner planets are rich in iron and other heavy elements of the kind needed for life. And then we have Pluto, Sedna, and the Kuiper Belt of asteroids beyond Neptune. Astronomers think they are similar to the Earth-like planets. We have the Sun and the giant planets made of one kind of material, with an inner and outer band of planets made of another kind of material.
Another idea is that the Sun gradually captured different kinds of planets. That would explain their diversity, but not why they formed groups. Why did four of the largest rocky planets go to the inside close to the Sun, when the rest of the rocky planets stayed outside the ring of the four giant gas planets? Once the planets are captured, initially they are likely to have many highly elongated, overlapping elliptical orbits. Getting them all to settle down into nearly circular, nicely concentric, well-nested non-interfering orbits like those in our solar system is not easy.
The theory of planetary capture also founders on the problem of angular momentum. If the Sun captures a passing planet it must somehow dissipate the planet’s kinetic energy and angular momentum so the planet will fall into orbit around the Sun. The only way to dissipate the energy is to have the encounter in a star-forming, dusty region with multiple encounters and transfers of energy to other planets or stars that aren’t captured.
This is the basis of the accretion disk theory, the most popular theory now. Recent news articles have described work on accretion theories.[i] Modelers think that planets coalesce from a swirling disk of gas and dust around new stars. Yet they are hard put to explain why light-weight Sun-like elements coalesce in a middle ring between inner and outer rings of rocky elements. The middle ring has elements like those of the center. Unlike elements might go to the outside, but why would a good portion of them seek an intermediate place between the center and the middle ring?
In the solar system, the accretion disk theory only begins to work when the dust gathers into small lumps called planetesimals. In our system Jupiter has most of the angular momentum. As soon as modelers get the parameters right to explain the angular momentum of the inner planets out to Jupiter, they get stuck with too-long formation times for the outer planets.
Science devoted an issue to planetary systems a few years ago. People without scientific training can still generally understand the news articles and reviews that accompany and interpret the scientific reports.[ii] The news editors on page 65 refer twice to “chance.” “Focusing on the four inner or terrestrial planets of our system, Richard A. Kerr (p. 68) finds that modeling studies have given researchers a new respect for the role of chance in determining these planets’ structure and composition, and therefore their ability to support life.” Jupiter has four large moons that Galileo originally discovered with a small telescope. Some researchers take these moons as a model in miniature of a planetary system. “In striking parallel with the larger solar system, the Jovian system may have formed by chance, each world is unique, and Europa may harbor life.” (We will have many questions about life under Europa’s ice when we discuss the thermodynamics of life in a later chapter.)
Physics Today also devoted an issue to planetary diversity.[iii] There is special interest in extrasolar planets. The Sun is not the only star with planets. Astronomers have discovered more than 100 planets orbiting other stars. Methods reported in April 2004 could only discover giant planets, of the size of Jupiter, or Saturn, but not as small as Neptune. Jupiter’s mass equals 318 Earth masses. Saturn’s mass equals 95 Earth masses, or 0.30 of Jupiter’s mass. Neptune’s mass equals 17 Earth masses, or 0.054 of Jupiter’s mass. Reported extra-solar planets have from 0.11 to 17.5 Jupiter masses. Until recently the discovery of extrasolar planets gave great hope to those who seek other places in the universe where life like ours may flourish. Now the hope has waned.
Especially disappointing has been the discovery that many extrasolar planets come very close to their parent star. That makes them too hot for hydrocarbon-based life. Others are “wild,” with highly elliptical, eccentric orbits. They spend long periods of time far from their parent star in the dark and cold, but periodically rush in close to fry any precursor of life that may have formed. Our giant planets, Jupiter, Saturn, Uranus, and Neptune, are tame. They move sedately in nearly circular orbits that nest nicely within one another. This unusual arrangement protects the Earth and the other inner planets.
Less than one in a hundred of the extrasolar planetary systems discovered so far have their Jupiter thoroughly domesticated. The one known example is our own solar system. This fact has raised the question: Is the process that made our solar system very unusual? Is it markedly dissimilar to the process that made all the known extrasolar systems?
When astronomers first discovered extrasolar planets many scientists thought they were getting statistics on the occurrence of Earth-like planets. The methods that detected the first hundred extrasolar planets couldn’t detect a planet the size of Earth. But what if the Jupiter-like extrasolar planets were representative of extrasolar systems similar to ours? Now some astronomers have examined that assumption.[iv] According to their article there may be two mechanisms for planet formation. The mechanism most studied applies to our solar system. But there may be another mechanism that only forms giant gas planets. It appears that the second mechanism operates much more frequently, because the extrasolar planets already found have very different orbital characteristics from those of our Jupiter. They may not be good places to look for life at all. In that case we still have no statistics for the probabilities of finding a solar system like ours. One can’t estimate the average distance between systems similar to ours if we know of only one system. The occurrence of life may still be a miracle.
[i] Ahrens, Thomas J., “The Origin of the Earth,” Physics Today, 47 (Number 8‑Part 1, August 1994), pp. 38–45.
[ii] Kerr, Richard A., “Making new worlds with a throw of the dice,” Science, 286 (Number 5437, 1 October 1999), pp. 68–69.
[iii] Physics Today, 57 (Number 4, April 2004), pp. 43–83.
[iv] Beer, M. E., A.R. King, M. Livio and J. E. Pringle, “How special is the Solar System?” Monthly Notices of the Royal Astronomical Society, 000 (30 July 2004), pp. 1–6.