There's only one planet in our solar system that can communicate across interplanetary space. But imagine if things were different. Say if Mars, for example, had intelligent beings able to talk to us at the same time as we talked to them - how would that have changed our history?
This scenario is impossible in our own solar system, but could be (albeit remotely) possible elsewhere in the galaxy. This is part of what the University of Nevada, Las Vegas' Jason Steffen considered in a new paper looking at possibly habitable planetary pairs. It's not something that has been yet seen by the Kepler space telescope, although the NASA mission has spotted plenty of planets close to one other.
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"What we found was that the spin axes were not adversely affected by the nearby planet - meaning that the climates would be as stable as the climates of the solar system planets," Steffen wrote in an email to Discovery News.
"We also found that: 1) there is a significant increase in the frequency of material successfully transferring between the planets, 2) that any biological material on the collision fragments would have an increased likelihood of surviving the impact since the energy needed to make the transfer is much less than in the solar system, and 3) that if one piece of debris makes it to the other planet that there is an increased probability that several pieces of debris would also make it successfully."
The transference of rocks from one planet to another after asteroid impacts would increase the possibility of also transferring biological material. This could spread life throughout a planetary system - a hypothesis called "panspermia." But say if life takes hold and evolves on neighboring planets?
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A recent press release had Steffen speculating about communication between evolved intelligent beings on such planets, but when asked about it he said that the scenario is "improbable" - although he does stand behind the quote. In our own solar system, Earth is more than 4 billion years old and telescopes have only been in use for 400 years, or 0.00001 percent of that time.
"Now do that twice in the same system," he said, referring to Mars. "So, it's far more likely that a space-traveling civilization would find food to eat on the other planet. Nevertheless, it is possible that there could be intelligent life at a similar stage of development. In that case, telescopes from the 1700′s or early 1800′s would have been capable of seeing surface features on the other planet. At that point, it would just take some moderately large mirrors to make oneself known."
Even if the planets can't share communications, they possibly could share similar biological histories. We already know of meteorites that travel from Mars and impact the Earth. How often this is likely to happen, Steffen added, depends on how many asteroids there are in the system and where the asteroids are located.
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There are drawbacks to having planets living close to one other. For example, their orbits could be unstable, or one planet could influence the seasons on the other planet by messing with its axial tilt. They could also induce tides on each other, although Steffen points out an Earth-mass planet 40 times further from us than the moon would produce weaker tides than what we experience today.
The new "Independence Day" and "Star Trek" movie trailers, which show conflict with other aliens, also brings another question to mind: would two intelligent entities in the same solar system be able to work out their differences? Steffen says probably, drawing hope from Earth examples such as research on Antarctica, and international water treaties.
"These facts would lead me to believe that we could cooperate relatively well on interplanetary issues - not stirring up trouble and not putting the whole planet at risk for nationalistic or personal advantage. When I was younger, I would fight with my sisters. But, if someone else picked on one of them, that was a whole different situation."
Steffen's research has been accepted by the Astrophysical Journal, and is available on the preprint service Arxiv.