Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] is Venus the hottest planet, whereas Mercury is closer to the Sun? – Sejal M., age 7, Bangalore, IndiaWhen the solar system had just formed, 4.5 billion years ago, Venus was probably a tropical paradise. Oceans of water likely covered its surface, and puffy clouds dotted its skies. At that time, Mercury, the planet closest to the Sun, was undoubtedly the hottest planet in the solar system.But then the Sun grew brighter, and a series of events known as a runaway greenhouse effect caused Venus’s surface temperature to soar well past Mercury’s. To be sure, as the closest planet to the Sun, parts of Mercury are still extremely hot. Its surface temperature on its sunlit side is around 800 degrees Fahrenheit (430 degrees Celsius), hot enough to melt lead.AdvertisementAdvertisementI’m a space scientist with a passion for teaching physics and astronomy. I’ve seen Venus shine brightly in the western sky just after sunset, and, on rare occasions, seen Mercury briefly appear as an “evening star.”What is the greenhouse effect?Venus isn’t the only planet that has experienced a greenhouse effect. Today, the same phenomenon keeps Earth’s surface habitable.[embedded content]The greenhouse effect traps heat in a planet’s atmosphere.Warm objects emit electromagnetic radiation, or light, because their atoms and molecules are constantly vibrating. Earth’s surface is warmed by the Sun and gives off infrared light, which is invisible to your eyes. Humans and animals also glow in infrared, and most of the energy released by a campfire is infrared radiation. The hotter an object is, the more infrared radiation it emits.AdvertisementAdvertisementOn a planet without an atmosphere, such as Mercury, this energy escapes directly into space. As a result, there is an enormous difference in temperature between Mercury’s blistering hot sunlit side and its night side, where temperatures can drop to minus 290 degrees F (minus 180 degrees C).On Earth, greenhouse gases in the atmosphere, including molecules of carbon dioxide, methane and water vapor, absorb and trap some of this infrared radiation, acting like a blanket around the planet and keeping the surface warm.Earth’s average surface temperature is about 59 F (15 C). Without greenhouse gases, the temperature would be closer to 0 F (minus 18 C), and Earth would resemble a giant snowball. We definitely need a moderate greenhouse effect to make our planet habitable.Diverging fatesBut how could …
