Understanding Why Venus is Hotter than Mercury

New Delhi, India Jul 19, 2026 ALN: 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. However, a series of transformative events altered the fates of these two neighboring planets, leading to Venus becoming the hottest planet in our solar system despite its second-place position in proximity to the Sun.

As the Sun grew brighter over millions of years, a phenomenon known as a runaway greenhouse effect took hold on Venus, causing its 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. However, this extreme heat is offset by the fact that Mercury has no substantial atmosphere to retain heat during its night cycle, where temperatures can plummet to minus 290 degrees F (minus 180 degrees C). This stark contrast highlights the significant role that atmospheric conditions play in planetary temperature regulation.

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.” These observations not only showcase the beauty of our solar system but also serve as a reminder of the complex dynamics at play between these two planets.

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 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.

On 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. In contrast, Earth benefits from an atmosphere rich in greenhouse gases that 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 fates

But how could Venus, a planet that is nearly the same size as Earth and that was once made up of the same materials, end up so different from our home world? There are several clues to why Venus is such a hot, inhospitable planet today. Scientists agree that because of their similar sizes and proximity to the Sun, Earth and Venus likely started out with comparable amounts of water and carbon dioxide. However, their evolutionary paths diverged significantly.

On Earth, most of the carbon dioxide is dissolved in the oceans or locked away in rocks, such as limestone. Venus may have once stored carbon dioxide in a similar way. Scientific models of stars indicate that the Sun has grown about 40% brighter since the early history of the solar system. Four billion years ago, it emitted only about 70% of the energy it does today. This increase in solar output had profound implications for the planets in its vicinity.

Scientists continue to debate whether Venus ever had long-lived oceans. However, evidence suggests that the planet possessed large amounts of water early in its history. As the Sun’s brightness increased, more and more water would have evaporated into the atmosphere. Without surface water to absorb and store carbon dioxide, nearly all of this greenhouse gas accumulated in the atmosphere. This process, in turn, would have caused even more evaporation and further warming, creating a feedback loop known as a runaway greenhouse effect.

Today, carbon dioxide makes up about 96% of Venus’ atmosphere, and its surface temperature is a nearly constant 867 F (464 C) day or night. Venus is now stuck in this state, and its atmosphere and surface temperature will remain like this for a very long time. Imagine how thick Earth’s atmosphere would be if all the oceans evaporated and all the carbon dioxide dissolved in them were released into the air. Something very much like this happened on Venus. As a result, the atmospheric pressure at Venus’ surface is more than 90 times greater than Earth’s atmospheric pressure at sea level. To experience such a high pressure on Earth, you would have to dive to a depth of about 3,000 feet (940 meters). That much pressure is like having five elephants standing on your shoulders!

A runaway greenhouse effect on Earth?

You may have heard scientists warn that Earth could experience some of the same processes affecting Venus if human-caused climate change continues unchecked. Global temperatures are rising as people burn coal, oil, and natural gas for industrial activities and transportation, releasing large amounts of carbon dioxide into the atmosphere. Carbon dioxide stays in the atmosphere for a long time. As more and more accumulates there, it traps more heat, raising the temperature on the planet’s surface.

Another major greenhouse gas is methane. As Earth warms, an important source of methane is thawing permafrost, the permanently frozen ground found mostly in the Arctic. When the icy ground thaws, it exposes organic matter that had been frozen for thousands of years, including ancient plants, mosses, and even the remains of woolly mammoths and other large animals. Bacteria can then decompose that matter, and as they do, they release methane into the atmosphere. As Earth warms, more permafrost melts, releasing even more methane and creating a vicious cycle.

While Earth may never reach the extreme temperatures found on Venus today, our planet could become a much less comfortable place as temperatures rise. The implications of this could be profound, affecting weather patterns, sea levels, and ecosystems globally. However, Earth’s surface temperature will never reach anywhere near that of Venus in our lifetime, and Venus will remain the hottest planet in the solar system—despite being farther from the Sun than Mercury.

Understanding the contrasting fates of Venus and Earth not only provides insight into the history and evolution of our solar system but also serves as a cautionary tale about the importance of maintaining a balanced atmosphere. The lessons learned from Venus's extreme climate can inform our understanding of climate change on Earth and emphasize the need for sustainable practices to mitigate the impact of human activities on our planet's delicate climate system.