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How ‘rare’ is Earth?
PLANET Earth. Is it unique as a life-bearing planet or are there other similar rocky, wet and hospitable to life worlds out there in the universe?
Recently, virtual computer models, which were designed to look into the possibility of the existence of Earthlike planets elsewhere, produced interesting results showing that Earth-like planets are not an impossibility. The computer simulations came up with models of planets with orbits which were similar, and also planets with and without liquid water as well as other rocky planets.
Sean Raymond, who is a doctoral student in astronomy from the University of Washington, explains, “We found there’s a much wider possible range of masses and water content on terrestrial planets. You can have planets that are half the size of Earth and are very dry or you can have planets three times bigger than Earth, with ten times more water.” The whole idea behind the simulations which were done by Sean Raymond, besides an astronomy professor Thomas Quinn and Jonathan Lunine, a professor of planetary science and physics at the University of Arizona, was to determine if the four inner planets of our solar system are typical in their formation or whether they are the result of some extreme cosmic scenario. The results, which were announced in December, will be published in the journal Icarus.
Today, more than 100 planets have been detected by astronomers around other stars. They are gas giants and not places where life might reside. Theorists, on the other hand, are mathematically speculating that other rocky, Earth-mass planets might be lurking somewhere within those systems and that there is no reason to rule them out.
The simulations in the new model took into consideration the kind of rocky planets that might form around a sun, which has a Jupiter-like gas giant. These models though, are extreme scenarios and might not represent what the systems are actually like.
In each of the 44 simulations there is just one Jupiter, assuming that such a planet formed much earlier than other terrestrial planets. Some of the computer models have a bigger Jupiter and some have relatively the same mass.
The virtual Jupiters have both, circular and elliptical orbits. The interesting fact is that when the Jupiter is the same size as ours, a solar system like the one we live in, forms.
On the other hand, even small adjustments or differences in the starting conditions produce surprisingly different results. In one case, just one terrestrial planet formed which was four times larger than Earth and had half as much water. There was one model, which produced five small rocky planets which were all much smaller than Earth. In all the simulations at least one rocky planet seems to have evolved.
Where there is water
Can habitable planets commonly form around stars other than our Sun? The study also focused on the question of whether the formation of terrestrial planets was a common occurrence in the Universe. Since it has been agreed upon by scientists that water is the key element for life, it turned out an interesting picture. The presence of water seemed to be directly connected to the orbit of the Jupiter-like planet.
As Raymond explains, “The more eccentric giant planet orbits result in drier terrestrial planets. Conversely, more circular giant planet orbits mean wetter terrestrial planets.”
The reason for this scenario is that a giant planet which has a circular orbit tends to send water-laden comets towards the rocky inner planets and the Jupiters with elliptical orbits tend to send them outward.
Experts think that Earth was dry when it was initially formed and water was delivered by the water-laden asteroids and comets that struck it later on. These are thought to have formed away from the Sun in order to sustain the water within. Our own Jupiter has a slightly elliptical orbit; this makes sure that Earth is a well-balanced world. It is neither a fully liquid water world nor a dry desert.
The candidates
Out of the hundred or more stars that have been discovered with extrasolar planets, where do we have a chance of finding a habitable world? Most of these planets are too close to their parent stars in order to have Earth-like orbits and to support life. Barrie Jones and Nick Sleep of the Open University in England sorted out five star systems in which they tossed in an Earth-like planet in their computer models. They were: Our Solar System, 51 Pegasi, 70 Virgini and 47 Ursae Majoris. Two of the systems did not support the introduction of a terrestrial and habitable world and the computer simulations discarded the planets.
In the other three, the results were different. In an email interview to SPACE.com, Sleep says, “We had no expectations of the other three. But in fact in all of them, Earth-mass planets could be in stable orbits in at least some of each habitable zone.”
This method of looking for extrasolar planets is the most feasible one at the moment as researchers feel that detecting Earth-like planets around other star systems is not that easy and knowing which are the possible habitable zones could save a lot of time.
“Our work should identify the exosystems where one is more likely to find ‘Earths,’ and so our work should help in target-selection. If no Earths are found in habitable zones where our work indicates they could exist, then that should be of interest to people trying to model the formation of exosystems,” says Jones.
The purpose of this effort is to look for life in other parts of the Universe. The systems which have been discovered so far look very different from ours but that, researchers feel, could be due to the fact that only large planets can be detected with present technology. However, experts feel that the next decade might give us more fruitful results.
Many other factors may contribute to the existence of life as we know it. Jones points out that some experts feel that the shifting of the crust, which is the cause for earthquakes, recycles the stuff needed and supports the existence of life.
In order to stabilize the rotation of the future life-bearing planet, a moon is required to avoid large climatic changes. But Jones says that he is not sure whether that is essential. Researchers also argue that a Jupiter-like planet in the outer limits is also needed to shield the planet from constant comet and asteroid bombardment.
All this is an assumption so far and we can only base computer simulations on this as we have no direct observations. But so far the picture is optimistic.
“Our biggest assumption is that Earth-mass planets can form in the exosystems. Studies by others are beginning to show that this is possible, at least in some of the exosystems,” says Jones.
Out of all the systems looked at by the researchers, 47 Ursae Majoris is an agreeable contender. It is a Sun-like star but a little older. Since it is hotter and brighter, it’s habitable zone lies farther out than ours. There are two giant planets that orbit it. The inner one is a little farther than Mars is from the Sun and is 2.5 times larger than Jupiter.
The other one is on the outer side and is about the same size as Jupiter. There is a possibility that other, terrestrial planets might orbit the system but it is not possible to find them with the technology we have today. Jones says, “It’s certainly a system worth exploring for an earthlike planet and for life.”
With the discovery of more and more star systems, astronomers need to narrow down on the possible contenders which stand a chance of being Earth-like.
“I think we will shortly discover systems with the giants further out, more like where Jupiter is in the solar system. We will then see whether Earth-mass planets could exist in their habitable zones,” says Jones.
Astronomers are also hopeful that if the galaxy has planets in abundance then those planets could also have their own moons. And if the moons are in the right place at the right time, they could be hospitable to life. The downside is that since none of these sub-worlds have been observed, only theoretically assumed, they cannot be studied.
At present there are loftier targets to look for as scientists focus their attention on Earth-like planets. There is also the fact that there are moons in our own solar system that need to be studied and are much easier to observe. But the fact remains that all these mysterious sub-worlds might be hosts to life and have atmospheres capable of supporting it. Peter Ward, co-author of the book, Rare Earth, and professor of geological sciences at the University of Washington, states, “moons form so commonly in our solar system that it would be ludicrous to think that this is unique.”
There is a parade of a 100 moons orbiting planets in our own solar system. There are 39 around Jupiter, 30 orbit Saturn, Uranus has more than 20, Neptune has 8, there are two around Mars, one around Pluto and lastly our own Moon. All these and more still get discovered each year as observation techniques improve.
With so many extrasolar planets discovered, mere speculation can be done about the number of moons that might be out there. If there are 300 billion stars in the Milky Way and 10 per cent are like the Sun and have planets, then the study estimates 30 billion Jupiters and the same number of Earth-like planets. Even if we do not take other possible planets into consideration and if we only count the four large moons around Jupiter and one around Earth, we have 150 billion moons. And that is a very modest estimate. All this is just theory and numbers, not certainties but a possibility, nevertheless. According to Alan Boss, who is a top planet formation theorist from the Carnegie Institution in Washington, it is “highly likely” that some of the extra-solar planets, which have been discovered, have moons. This is based on the fact that every large cosmic object that astronomers can study has satellites orbiting it whether it is a planet or an asteroid and moons are a common phenomenon in the Universe.
In fact, the only round celestial object that does not have a moon is a moon itself. If a moon is in a region in a solar system where the sun’s energy is just right to help in generating a moderate temperature, like our Moon is, then it can support life.
Boss agrees with the idea: “moons orbiting planets with orbital radii near their star’s habitable zone should have a good chance of being habitable.”
Even though all the exo-planets discovered till now are gaseous, they could have orbiting moons which are rocky and hosts to life. Peter Ward, the author also agrees with the notion, but complex and intelligent life is another question altogether. He says, “the biggest challenge might be impact rate. Jupiter’s moons are blasted by asteroids and comets. Jupiter brings them in and boom.”
Even in the chaotic scenario, theorists feel that microbial life might find a way to hang in there. The most promising subject in our solar system is Jupiter’s moon Europa. Though it can not be expected that colourful fish might be swimming under Europa’s icy crust but even the presence of microbes would double the number of living worlds in our knowledge.
How a moon could be capable of supporting life depends on the fact that it can survive long enough to let evolution take hold. In a study done by Jason Barnes, a researcher with the University of Arizona, with his colleague, David O’Brian, it is stated that a large planet that orbits too close to its star cannot support its moon over billions of years and the moons tend to “crash into the planet.”
Even though Jupiter has had its moons for a long time, there is not much solar radiation required for life in that region. So where can a planet be in order to have a moon long enough to have life on it?
According to Barnes, the “critical minimum location” would be about half the distance of Earth from the Sun. Meaning that it is theoretically possible that a large planet that orbits its host star at about one Earth distance from the Sun, which would be a comfortable distance where the radiation is in the right amount, would have a rocky habitable moon. Maybe another Earth will be discovered in our lifetimes with the state-of-the-art telescopes and technology being applied to satisfy mankind’s search for life in the Universe.
The writer
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