There are two theories as to how planets in the solar system were created. The first and most widely accepted, core accretion, works well with the formation of the terrestrial planets like Venus but has problems with giant planets. The second, the disk instability method, may account for the creation of giant planets. Scientists are continuing to study planets in and out of the solar system in an effort to better understand which of these methods is most accurate.
The core accretion model
Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.
With the rise of the sun, the remaining material began to clump up. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create smaller terrestrial worlds like Venus. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants. In this way, asteroids, comets, planets, and moons were created.
Like Earth, the rocky core of Venus formed first, then began to gravitationally capture the lighter elements that would make up its atmosphere. Early in its life, Venus may have had an atmosphere much like Earth holds today. Studies have shown that if water on a young Venus evaporated billions of years ago, the greenhouse levels in the atmosphere would have escalated, giving rise to a runaway greenhouse effect that significantly increased the planet's temperature. Today, thanks to the excess of carbon dioxide and traces of nitrogen, the surface of Venus is hot enough to melt lead.
The disk instability model
Although the core accretion model works fine for terrestrial planets, gas giants would have needed to evolve rapidly to grab hold of the significant mass of lighter gases they contain. But simulations have not been able to account for this rapid formation. According to models, the process takes several million years, longer than the light gases were available in the early solar system. At the same time, the core accretion model faces a migration issue, as the baby planets are likely to spiral into the sun in a short amount of time.
According to a relatively new theory, disk instability, clumps of dust and gas are bound together early in the life of the solar system. Over time, these clumps slowly compact into a giant planet. These planets can form faster than their core accretion rivals, sometimes in as little as a thousand years, allowing them to trap the rapidly-vanishing lighter gases. They also quickly reach an orbit-stabilizing mass that keeps them from death-marching into the sun.
As scientists continue to study planets inside of the solar system, as well as around other stars, they will better understand how Venus and its siblings formed.
— Nola Taylor Redd, SPACE.com Contributor