Which planet's poisonous atmosphere is attributed to the "runaway greenhouse effect?"
And the answer: Venus.
The second closest planet to the sun, Venus is also the hottest and brightest planet in our solar system. It has a toxic atmosphere filled with carbon dioxide and is shrouded in clouds of sulfuric acid.
Venus is one of the most unique planets in our solar system. Not only is its atmosphere toxic, hot and barren, but it is a planet which takes longer to rotate on its axis than it does around the sun. Effectively, this means that a day on Venus is longer than a year. On top of that, Venus is hotter than neighboring planet Mercury— which is closer to the sun. This is due to the high amount of carbon dioxide which gets trapped in the atmosphere.
But wait: the Venus fun facts don't end there. This peculiar planet doesn't just rotate around its axis slowly— it actually rotates in a direction unlike every other planet in our solar system. While all other planets spin counterclockwise on their axis and orbit the Sun in a counterclockwise direction, Venus orbits the Sun counterclockwise... and spins clockwise! This unusual axis rotation is actually due to the fact that the planet is upside down– it was knocked off its upright position earlier in its history! The only other planet to spin in a strange direction is Uranus, which spins on its side. Both of these events are believed to be the result of another collision early on in the planets' life.
Venus is also the second-brightest natural object in the night sky, after the Moon. It turns out that those clouds of sulfuric gas actually do serve some good— the planet is able to reflect light with ease, making it appear bright and shiny in the night sky. For this reason, Venus was the first planet to have its motions plotted across the sky as far back as 2 BCE.
Learn more about Venus here.