The rocky planets

A pale, whitish-yellow globe, covered in faintly banded clouds, against a black background. — Venus, a planet the size of the Earth entirely wrapped in a thick layer of clouds, photographed by NASA’s Mariner 10 probe in February 1974. NASA/JPL-Caltech, Public domain, source

Which is the hottest planet in the Solar System?¹

Almost everyone answers Mercury, because it is the closest to the Sun.

It is Venus. Yet it is much further away.

Why

On the surface of Venus, the temperature is about 467 °C. Hot enough to melt lead.²

Mercury is almost twice as close to the Sun, yet it is not as hot.

The reason fits in one sentence:

Mercury has practically no atmosphere. The heat it receives from the Sun goes straight back out into space, like a stone that cools as soon as it is taken out of the fire.

Venus is wrapped in a thick atmosphere that traps heat. Beneath its clouds of sulphuric acid, the ground heats up and can no longer cool down: this is a runaway greenhouse effect.³

So it is not distance that decides. It is what lies around the planet.

A surprising consequence

On Earth, it is hotter at the equator than at the pole, and hotter by day than by night. That seems normal.

On Venus, no respite is to be expected. No season to cool things down: the heat never lets up.⁴

The days, meanwhile, are bafflingly long. A day on Venus lasts 243 Earth days — longer than its year, which lasts only 225. From sunrise to sunset, 117 Earth days go by. What is more, the Sun rises there in the west: the planet spins backwards.⁵

Interactive

One turn on its axis

The Earth turns once on its axis in a little under 24 hours. Each of the other planets has its own rhythm.

The question

Which planet takes the longest to turn once on its axis?

Without JavaScript, here are the animation’s data.

The eight planets: one turn on their axis, and the tilt of their axis
Planet	One turn on its axis (hours)	Direction	Tilt of the axis (degrees)	Source
Mercury	1,407.6	forwards	0.034	1
Venus	5,832.5	backwards	177.4	1
Earth	23.9	forwards	23.4	1
Mars	24.6	forwards	25.2	1
Jupiter	9.9	forwards	3.1	1
Saturn	10.7	forwards	26.7	1
Uranus	17.2	backwards	97.8	1
Neptune	16.1	forwards	28.3	1

What the article says
Fact	Value	Source
A day on Venus	243 Earth days	2
A year on Venus	225 Earth days	2

NASA marks a backwards (retrograde) rotation with a minus sign; the table spells it out. The planets’ sizes are not to scale, and their tilts are drawn in the plane of the figure.

Mercury, the Earth, Mars and Jupiter are maps of their surface wrapped round a globe. Venus, Saturn, Uranus and Neptune are still photos: only their white marker turns.

The takeaway Venus is by far the slowest: one turn on its axis takes 243 Earth days, longer than its year. It also spins backwards, as Uranus does. The giants, by contrast, turn once in less than an Earth day.

Photo credits 8

Global mosaic of Mercury’s surface, in black and white, made from images taken by the MESSENGER spacecraft (equirectangular projection). NASA/Johns Hopkins University Applied Physics Laboratory, Public domain, source
Venus, wrapped in clouds, photographed by NASA’s Mariner 10 probe in February 1974. NASA/JPL-Caltech, Public domain, source
Blue Marble: Next Generation, with land topography and sea-floor bathymetry, in equirectangular projection: map produced by Reto Stöckli, NASA Earth Observatory (NASA Goddard Space Flight Center). Reto Stöckli, NASA Earth Observatory, modified by NordNordWest, Public domain, source
Global map of Mars, the MDIM 2.1 colour mosaic made from Viking Orbiter images (equirectangular projection). NASA / Jet Propulsion Lab / USGS, Public domain, source
Cylindrical map of Jupiter, made from images taken by the Cassini spacecraft on 11 and 12 December 2000; its colours are close to those the human eye would see (PIA07782). NASA/JPL/Space Science Institute, Public domain, source
Saturn in natural colour, from images taken by the Cassini probe (2008). NASA / JPL / Space Science Institute, Public domain, source
Uranus photographed by the Voyager 2 probe in 1986. NASA/JPL-Caltech, Public domain, source
Neptune, from the last whole-planet images taken by Voyager 2 through green and orange filters (1989); the Great Dark Spot is visible. NASA, Public domain, source

Sources 3

Williams, David R., “Planetary Fact Sheet – Metric”, NASA Goddard Space Flight Center (NSSDCA), updated 18 March 2025, rows “Rotation Period (hours)” and “Obliquity to Orbit (degrees)”, columns Mercury to Neptune (Moon and Pluto excluded).
Rotation Period (hours) 1407.6 -5832.5 23.9 655.7 24.6 9.9 10.7 -17.2 16.1 -153.3 […] Obliquity to Orbit (degrees) 0.034 177.4 23.4 6.7 25.2 3.1 26.7 97.8 28.3 119.5
Open access open the source · checked on 28 October 2026
NASA, “Venus: Facts”, section “Venus: Facts”.
your "day" would be 243 Earth days long — longer even than a Venus year (one trip around the Sun), which takes only 225 Earth days
Open access open the source · checked on 9 September 2026
Williams, David R., “Planetary Fact Sheet Notes”, NASA Goddard Space Flight Center (NSSDCA), updated 20 December 2021, definition of “Rotation Period (hours)”.
Negative numbers indicate retrograde (backwards relative to the Earth) rotation.
Open access open the source · checked on 28 October 2026

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What it took to go and see

Now imagine what it takes to land a machine there: 467 °C and a pressure of about 93 times that of the air at sea level — the equivalent of a dive to more than nine hundred metres underwater.⁶

In the 1970s and 1980s, Soviet probes of the Venera programme managed it. None lasted very long: the heat and the pressure eventually crush everything.

Shorter than a film

Here is the fact we find the most striking on this page.

No probe that has landed on Venus has survived there much more than an hour. The length of a feature film, and then the machine falls silent.⁷

The few photographs taken on the surface of Venus were therefore taken in very little time. They are almost all we have: no one has gone back there with a camera since.

And why this programme remains so little known

This programme is one of the great technical achievements of the twentieth century. Landing a machine that works inside a pressurised oven is a feat of engineering.

Yet it remains fairly little known, at least in Europe and America. The reasons can be identified.

First, because it took place in a language that most Western scientists did not read. Second, because it belonged to a state that no longer exists, and that therefore has far fewer people to tell its story.

This is a property of knowledge, and it holds everywhere:

What is not read is not cited, and what is not cited gradually disappears from the story.

This idea appears elsewhere on this site, in connection with history: in a conflict, it is the victor’s archives that survive. Here there is neither victor nor conflict — just a language barrier and a vanished country. The result is comparable.

Did Venus have oceans?

Two incompatible accounts clash:

“Wet Venus”: the surface was at first temperate enough to hold liquid water; then a runaway greenhouse effect, fuelled by intense volcanic activity, boiled it all away.
“Dry Venus”: the planet was born hot, and liquid water has never been able to condense on its surface.⁸

How people are trying to settle it

Tereza Constantinou, a PhD student at Cambridge’s Institute of Astronomy, who authored this study with Oliver Shorttle and Paul Rimmer, frames the problem well:⁹

“Both of those theories are based on climate models, but we wanted to take a different approach based on observations of Venus’ current atmospheric chemistry.”¹⁰

Two competing models, and someone who goes looking for a different kind of data to decide between them.

The reasoning is elegant, and accessible to anyone. For an atmosphere to remain stable, everything removed from it must be put back: a planet’s interior and exterior are in constant chemical communication. So the team calculated how fast three molecules — water, carbon dioxide and carbonyl sulphide — are destroyed in the air of Venus. What the volcanoes emit must exactly offset these losses. And what the volcanoes emit comes from the interior.¹¹

The result: the volcanic gases of Venus contain at most 6% water vapour. On Earth, eruptions are mostly steam, because our interior is rich in water.¹²

A dry interior cannot have supplied oceans.¹³

The approach deserves to be highlighted: no one looked at the past. The present was measured, and from it was deduced what the past could not have contained.

But it is not settled

This is the part that popular retellings leave out.

The other camp has not conceded, and the team itself does not present the matter as closed. Constantinou says so: “We won’t know for sure whether Venus can or did support life until we send probes at the end of this decade.”¹⁴

The American DAVINCI mission, which is to fly past the planet and drop a probe into its atmosphere, will be able to put this conclusion to the test.¹⁵

A sentence we wanted to quote

Here is what she said when presenting her result:

“We would have loved to find that Venus was once a planet much closer to our own, so it’s kind of sad in a way to find out that it wasn’t, but ultimately it’s more useful to focus the search on planets that are mostly likely to be able to support life – at least life as we know it.”¹⁶

A researcher who says she would have preferred the other answer — and who publishes the one her data give.

This may be the most useful sentence on this whole page.

The four rocky planets

Mercury, Venus, Earth and Mars are the four planets closest to the Sun. They are called terrestrial planets because they have solid surfaces.¹⁷

Mercury is the smallest planet in the solar system and the closest to the Sun. It goes round the Sun in 88 Earth days. By day its surface can reach 430 °C; at night, with no atmosphere to hold the heat, it drops as low as −180 °C. It has no moon.¹⁸

Mars is about half the size of the Earth. Its red colour comes from rust: the iron in its rocks, soil and dust has oxidised. Its very thin atmosphere is made mostly of carbon dioxide. It bears the largest volcano in the solar system, Olympus Mons, and two small moons, Phobos and Deimos. Mars seems to have had a watery past: ancient river valleys, deltas and lakebeds can be seen there, as well as rocks that could only have formed in liquid water.¹⁹

The Earth has its own pages: see Entry 1.6 The Earth and the Moon, and the whole of domain 7, Earth and climate, for its interior, its atmosphere and its climate.

Note 1

making it the hottest planet in our solar system

NASA, “Venus: Facts”

Why