Modern-day Venus is a hellish place, but that wasn't always the case. Studies have shown that billions of years ago it might have been just as habitable as Earth. So what happened? New research has found that if ancient Venus was home to vast oceans, tidal drags may have slowed down the rotation of the planet and let it bake into the lifeless world we know today.

Despite its nickname as Earth's sister planet, Venus currently isn't very homely. A suffocating atmosphere composed of over 96 percent carbon dioxide (compared to Earth's 0.04 percent), complete with clouds of sulfuric acid that blanket a rocky, sterile surface where temperatures average 462° C (863° F).

This unpleasantness is partly thanks to the fact that the planet rotates extremely slowly – one Venusian day is as long as 243 Earth days, or about two-thirds of an Earth year. That means that any given spot on the surface is exposed to constant sunlight for months at a time.

But it probably wasn't always so horrible. NASA missions to the planet have revealed clues that, like Mars, Venus could have once been home to oceans of water billions of years ago. Back then, the atmosphere would have been less smothering and the planet itself probably spun much faster. All of these factors could have made ancient Venus a veritable Eden.

So what changed? According to the new research from scientists at NASA, Bangor University and the University of Washington the cause is, ironically, the ocean itself.

Ocean tides are known to gradually slow down the rotation rate of a planet, thanks to friction between the currents and the seafloor. For instance, here on Earth this process is making the day about 20 seconds longer every million years or so. If Venus was once as watery as Earth, a similar process would have been happening, although these tidal effects would have been caused by the Sun, since Venus has no moons.

So the team on the new study set out to investigate just how this scenario might have played out. They ran several simulations of an ocean-laden Venus, varying the depth of the virtual seas and the rotational speeds of the planet, ranging from its current rate of 243 Earth days up to 64.

They found that the tides in an ocean on Venus would have been large enough to slow down the rotation of the planet, although the rate of that slowing-down would have varied depending on the ocean depth and how fast the planet was spinning to begin with.

In the most extreme cases, that tidal force could have slowed down the rotational period by as much as 72 Earth days per million years. That means the planet could have slowed to its current spin rate within 10 to 50 million years – a very short time frame, on the grand scale of things.

In turn, all that extra sunlight would have evaporated the ocean in that time, rendering the planet uninhabitable pretty quickly and leaving us with the harsh Venus we know today.

"This work shows how important tides can be to remodel the rotation of a planet, even if that ocean only exists for a few 100 million years, and how key the tides are for making a planet habitable," says Mattias Green, lead author of the study.

Interestingly, there are some scientists who believe that for all its foibles, modern-day Venus could still host life. Mysterious dark spots in high-altitude clouds could be microscopic forms of life similar to algae blooms here on Earth.

The new study was published in the Astrophysical Journal Letters.