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 CuriousKidsUS@theconversation.com.

Why is Venus the hottest planet, whereas Mercury is closer to the Sun? – Sejal M., age 7, Bangalore, India

Why is Venus the hottest planet, whereas Mercury is closer to the Sun? – Sejal M., age 7, Bangalore, India

When 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.

I’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.

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........

© The Conversation