Why Is Venus So Hot? The Runaway Greenhouse Effect Explained

Venus is so hot, about 465 °C at the surface, because its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect. That makes it hotter than Mercury, even though Mercury is much closer to the Sun. Venus is almost the same size as Earth, which is why scientists call it our twin, but its air is about 96.5 percent carbon dioxide and its surface pressure is about 92 times Earth’s.

This guide explains how the greenhouse effect works, how Venus may have ended up like this, what the surface is like, and the missions that are heading back.

Venus vs Earth at a glance

	Venus	Earth
Width	About 95% of Earth’s	12,742 km
Surface temperature	About 465 °C	About 15 °C
Surface pressure	About 92 bar	1 bar
Carbon dioxide in the air	About 96.5%	About 0.04%
Sunlight received	About 1.9 times Earth’s	1
Year length	225 Earth days	365 days
Day length (one spin)	243 Earth days	24 hours
Moons	0	1

Venus has no moons, as you can see in our guide to how many moons each planet has.

Explore Venus

The widget has two parts. First, ride down through Venus’s atmosphere and see how temperature and pressure change. Second, step through the leading explanation for how Venus got this way.

Venus: why the closest twin of Earth became an oven

Ride down through Venus’s atmosphere, then step through how the runaway greenhouse effect may have happened.

1. Descend through the atmosphere

For comparison, Earth at sea level: about 15 °C and 1 bar.

2. How did Venus get this way?

Back Next step

Temperatures and pressures at the stops are rounded values from published atmospheric models and spacecraft measurements. The steps in part 2 describe the leading scientific explanation. The details, especially whether Venus once had oceans, are still debated. Sources: NASA and ESA’s Venus Express.

How does the greenhouse effect work?

Sunlight passes through a planet's atmosphere and warms the ground. The warm ground then gives off heat as infrared radiation. Certain gases, such as carbon dioxide and water vapour, absorb some of that heat and send part of it back down. This is the greenhouse effect, and it keeps Earth about 33 °C warmer than it would be without an atmosphere, which makes life possible.

On Venus, the effect is extreme. A very thick blanket of carbon dioxide traps nearly all the heat, so the surface cannot cool down.

What is a runaway greenhouse effect?

A runaway greenhouse effect happens when warming triggers more warming, and the loop does not stop. Here is the sequence scientists think happened on Venus:

1. The planet gets hotter, for example because the Sun slowly brightens.
2. More water evaporates from the oceans.
3. Water vapour is a strong greenhouse gas, so the planet warms even more.
4. Eventually the oceans are gone and the heat has nowhere to go.

This is a hypothesis built on climate models and spacecraft data. It is the leading explanation, but the details are still debated.

How did Venus end up this way?

It started as a twin of Earth. Venus is made of similar rock and is about 95 percent as wide as Earth. Climate models suggest it may once have had liquid water on its surface. No one has found direct proof of ancient oceans, so this remains an open question.

The Sun grew brighter. The young Sun was dimmer than it is today and has slowly brightened. Venus already receives about 1.9 times as much sunlight as Earth, which makes it far more vulnerable to overheating.

Water was lost to space. In the upper atmosphere, sunlight split water vapour into hydrogen and oxygen, and the lightweight hydrogen escaped. Venus's remaining hydrogen contains over 100 times more of the heavy form, deuterium, than Earth's does. That is what you would expect if a lot of water was lost.

Carbon dioxide built up. Volcanoes keep releasing carbon dioxide. On Earth, oceans and rocks absorb a large share of it over time. Without oceans on Venus, the gas stayed in the air and piled up until it formed the dense atmosphere we see today.

Why is Venus hotter than Mercury?

Mercury is closer to the Sun but has almost no atmosphere, so it cannot hold heat. Its daytime temperature can reach about 430 °C, and at night it plunges far below freezing. Venus holds its heat day and night, so the surface sits at about 465 °C everywhere. That makes it the hottest planet in the solar system.

It may surprise you that Venus actually absorbs less sunlight than Earth does. Its thick clouds reflect about three quarters of the light that hits them. The heat comes from the greenhouse effect, not from how much sunlight reaches the surface.

What is the surface of Venus like?

The surface is a hot, dim, rocky world. The pressure is about 92 times that of Earth's, similar to being roughly 900 metres under the ocean. The temperature is high enough to melt lead.

• Clouds: Thick clouds of sulfuric acid cover the whole planet at 45 to 70 km up.
• Winds: Winds near the cloud tops reach about 360 km/h and circle the planet in roughly four Earth days.
• Slow spin: A day on Venus is longer than its year, and the planet spins backward compared with most others, so the Sun rises in the west.
• Volcanoes: Venus has thousands of volcanoes and vast lava plains. Radar images from the Magellan spacecraft hint that some may still be active.

The Soviet Venera landers made the first landings on another planet. Venera 7 sent signals from the surface in 1970. Venera 13 survived about two hours in 1982 and sent back the first colour pictures. No lander has lasted longer.

Could life exist on Venus?

The surface is far too hot for any life we know. A higher layer of the clouds, around 50 to 55 km up, has comfortable temperatures and pressure, and some scientists have wondered whether microbes could survive there. This is speculation. In 2020, a team reported a possible chemical signature of phosphine in the clouds, but other scientists disputed the result and it has not been confirmed.

For how researchers think about life beyond Earth, see our astrobiology section and the habitable zone guide.

Could Earth become like Venus?

Climate scientists generally say that Earth will not turn into Venus because of the warming we are causing now. Earth is farther from the Sun, and a true runaway greenhouse effect would need much more heating than current emissions can produce. In roughly a billion years, as the Sun keeps brightening, Earth's oceans could eventually be at risk.

Venus still matters for climate science because it shows how a planet's atmosphere can change the climate. Researchers use it to test climate models.

What missions are going back to Venus?

Venus has been quiet for a while. Japan's Akatsuki orbiter, which studied the planet for about a decade, lost contact in April 2024, and no spacecraft is orbiting Venus now. Several missions are planned, though their dates keep shifting with budgets and schedules:

• DAVINCI (NASA): A probe designed to drop through the atmosphere and measure its chemistry. Launch is currently planned around the end of 2030.
• VERITAS (NASA): An orbiter that will map the surface with radar. Launch is currently planned for late 2032.
• EnVision (ESA): A European orbiter planned for the early 2030s that will study the surface and atmosphere.

These three missions are sometimes called a "decade of Venus". Check each mission's official page for the latest dates before you rely on them.

Frequently asked questions

Key takeaways

• Venus is about 465 °C because of a runaway greenhouse effect and a very thick carbon dioxide atmosphere.
• It is hotter than Mercury and equally hot day and night.
• Venus probably lost its water to space, but whether it once had oceans is still unproven.
• Earth is not expected to follow Venus's path because of current warming.
• Three missions are planned for the early 2030s, though dates may change.

Continue exploring in our Solar System guide.

Sources

• NASA: Venus
• ESA: Venus Express
• NASA's Venus plans: DAVINCI and VERITAS schedule update
• SpaceNews: VERITAS and EnVision timing