In 2018, scientists found evidence for a subsurface lake using radar on the European Space Agency’s Mars Express spacecraft. Our robotic space missions there have found evidence of ancient lakes, and minerals that could only have formed in water. Mars, which is too far from the Sun to be in the habitable zone, once had flowing liquid water. The escape of liquid water from Venus’s surface may have directly led to the planet’s current inhospitable, dense atmosphere. Venus may have once had oceans, but its proximity to the brightening Sun caused the liquid water to evaporate. However, evidence suggests that the Sun used to be much dimmer. Mercury and Venus are not in the habitable zone because they are too close to the Sun to harbor liquid water. Earth is the only planet in our solar system’s habitable zone. While each planet in our solar system is unique, the 8 planets can generally be grouped into two different categories: the inner rocky planets (Mercury, Venus, Earth, and Mars) and the outer gas giants (Jupiter, Saturn, Uranus, and Neptune). An exoplanet orbiting a cooler star has to be much closer than an exoplanet orbiting a hotter star for liquid water to exist. That’s because stars have wide ranges of temperatures. However, it’s important to note that an exoplanet at this distance from its star might not necessarily be in the habitable zone. We sometimes call these exoplanets “Goldilocks” worlds because they’re not too hot, and not too cold.Įarth’s average distance from the Sun is 149,600,000 kilometers (92,900,000 miles), which is also called an astronomical unit (or 1 AU). Life as we know it requires liquid water, so exoplanets in their stars’ habitable zones are compelling places to search for life. While we love studying all of these worlds, scientists are particularly (and understandably) interested in the ones that could support life. Currently, we’ve identified over 4,000 exoplanets, and there are likely many more. One of the most exciting space discoveries of the past few decades is the existence of exoplanets, planets that orbit other stars.
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