Surprisingly, the slow, protracted growth of Earth, the interaction between Jupiter and Saturn, and the giant impact that formed our Moon<\/a> may have been decisive factors in making a world that is hospitable to living beings.<\/p> But, as astronomers are finding out, there are some very weird exoplanets<\/a> out there. When it comes to searching the Universe beyond the Solar System for signs of life or habitable worlds, as Morbidelli says: \u201cYou need some luck.\u201d<\/p> Will we ever find life beyond Earth<\/a>? Does an exoplanet’s atmosphere indicate the presence of life<\/a>?<\/p> These are fascinating questions filled with many possible answers, but what makes a planet habitable?<\/p> In the search for exoplanets capable of supporting life, astronomers could be helped by looking for 7 key features.<\/p> Obviously, Earth needs to orbit in the Sun\u2019s habitable zone. Many exoplanetary systems have planets much closer to their star, since young, full-grown planets tend to migrate inwards while embedded in the protoplanetary disc.<\/p> Earth did not: it grew slowly, remaining too small to migrate during the disc\u2019s lifetime, probably because Jupiter blocked dust inflow from the outer Solar System.<\/p> Then again, if Jupiter had migrated inwards, Earth would still have ended up close to (or even in) the Sun, or it might have been kicked out into interstellar space.<\/p> The fact Jupiter didn\u2019t migrate is likely to be due to the gravitational influence of a second giant planet in the system \u2013 Saturn.<\/p> Most exoplanet giants orbit their parent stars on elongated orbits. If that was the case for the giants in our Solar System, Earth\u2019s orbit would have become eccentric too.<\/p> A stretched-out orbit means a varying climate, which could prevent the evolution of complex organisms, and even the origin of life.<\/p> We\u2019ve been lucky that Jupiter remained on a rather circular path, despite its complex interactions with other planets.<\/p> Imagine the tilt of Earth\u2019s axis varied by many tens of degrees over time spans of a million years or so.<\/p> We know that\u2019s what happens to Mars<\/a>, and these huge changes in axial tilt angle may have contributed to the loss of most of the Red Planet\u2019s atmosphere and water.<\/p> Without Earth\u2019s more or less stable obliquity, life might have become extinct long ago, or it may never even have started.<\/p> So what prevents our planet\u2019s axis from flipping in every possible direction? It\u2019s the stabilising influence of its large Moon. Next time you look at its pockmarked face, say thanks.<\/p> Life needs water \u2013 at least life as we know it. Most of Earth\u2019s water probably arrived after the planet\u2019s formation, by impacting asteroids and (to a lesser degree) comets.<\/p> But fortunately, the amount of water brought in was relatively small.<\/p> If Earth had become a water world, a deep layer of high-pressure, high-density \u2018tetragonal\u2019 ice would have formed at the bottom of the ocean, separating the liquid water from the mineral-laden crust.<\/p> Without erosion-driven organics from the crust, the ocean would have remained sterile.<\/p> The solar nebula was more than 70% hydrogen. If Earth had formed rapidly, it would have accreted a thick, dense atmosphere of hydrogen (and helium), ending up resembling the many \u2018mini-Neptunes\u2019 found among exoplanets.<\/p> Life may have formed on such a world, but the oxygen produced by the first cyanobacteria would then combine with hydrogen to form water, and without an oxygen-rich atmosphere many complex life forms (humans included) would never evolve.<\/p> Apparently, Earth formed slowly and relatively late, after much of the solar nebula had already dissipated. Lucky us.<\/p> Thanks to Earth\u2019s plate tectonics and associated volcanism, our planet has been able to regulate its climate, despite the fact that the Sun was much fainter long ago and has increased in brightness ever since.<\/p> The well-known greenhouse gas CO2<\/sup> (carbon dioxide) is brought into the atmosphere by volcanoes, but washed away by rain storms, which are more frequent in warmer climates.<\/p> Eventually, plate tectonics returns CO2<\/sup> back into Earth\u2019s lithosphere. Thus, the CO2<\/sup> cycle acts like a thermostat.<\/p> So why does Earth have plate tectonics while Venus<\/a> doesn\u2019t? It’s most likely because of the giant impact from which the Moon was born.<\/p>![\"What](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/01\/Exoplanet_surface-ea33c99.jpg\" "\\"What")
7 features that make a planet habitable<\/strong><\/h2>
Right distance from their star<\/h3>
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More or less circular orbit<\/strong><\/h3>
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Stable rotational axis<\/strong><\/h3>
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Some water, but not too much<\/strong><\/h3>
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No hydrogen-rich atmosphere<\/strong><\/h3>
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Plate tectonics<\/strong><\/h3>
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Magnetic field<\/strong><\/h3>
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