NASA's robotic exploration of the solar system is heavily focused on either finding life or at least potentially-habitable planets. To this end, one priority has been to follow the water: find water, find life right? But another important aspect in the search for otherworldly life is a better understanding of our solar system. Specifically, how did we get to the perfect spot for life to flourish? Recent research suggests a mystery planet may have played a role. That’s right, our solar system may have previously had a fifth gas giant that has since simply disappeared.Earth is in a sort of Goldilocks position in our solar system. It's neither too hot nor too cold. We're at the right spot for water to exist as a liquid. The sun's heat and radiation levels are strong enough to have affected the chemical structure of the planet and generate life without frying us with huge bursts of radiation.Our lucky position is due at least in part to the four gas giant planets: Jupiter, Saturn, Uranus, and Neptune. Astronomers generally agree that the gas giants formed billions of years ago out of a cloud of gas and dust that surrounded our Sun when it was a new star. When the solar system was young, these massive planets pulled and pushed everything else before settling into their current orbits.Understanding how the gas giants might have contributed to Earth's final position will help astronomers understand the role of other planets in life. It might even help us understand how we ended up where we are.There are loads of clues out there. Trans-Neptunian bodies known as Kuiper Belt objects and the moon's craters all hold secrets about the environment of our early solar system. But there's another way to find out what might have happened billions of years ago. Computer models can give us an educated hint. With information about our current solar system structure and what astronomers do know about how it formed, they can generate models of its earliest incarnations.Dr. David Nesvorny of the Boulder, Colorado-based Southwest Research Institute did just that, using computer simulations to explore what the solar system may have looked like four billion years ago.Nesvorny started his simulations with the current number of planets in our system: four terrestrials, four gas giants, and a host of dwarf planets and other small bodies. When he ran a simulation of the System's formation with these constants, the virtual evolution ended up at a solar system that didn't resemble ours at all. So he changed the starting conditions. He added a fifth gas giant. With this rather large addition, simulations were ten times more likely to generate a model like the system we call home.A fifth gas giant might be the key to explaining the order in our solar system. Nesvorny suggests there was once a fifth planet roughly the size of Neptune orbiting the Sun about fifteen times further than the Earth is. He suggests that, like Neptune, it was a cold, icy world. A little over four billion years ago, its gravitational pull shook things up in the young solar system, yanking and pushing things around during a period of general instability. Back then, nothing in the solar system was unmovable.Then our second-Neptune met Jupiter. Even a Neptune-sized planet couldn't hold its own against the failed star that became our biggest giant. Second-Neptune was likely whipped out of our solar system by Jupiter's formidable gravity and thus became an orphaned planet on its own wandering path.The recent discovery of orphaned ice planets wandering alone through interstellar space makes this a plausible scenario, but as always with the early universe, we can’t be totally sure. Nesvorny ran 6,000 simulations that supported the idea of a second Neptune, but unless someone finds a way to travel back in time four billion years and watch the solar system's formation, we'll likely never know for sure if a mystery giant went the way of Atlantis after helping shape our solar system.
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Here’s a “solar system family portrait” from NASA.
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