Looking Back at the Solar System's InfancyThis artist's conception shows the closest known planetary system to our own, called Epsilon Eridani. Observations from NASA's Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Our own solar system may have looked like this as the new Sun and planets formed beginning 4.5 billion years ago. NASA/JPL-Caltech
The story of how the solar system-the Sun, planets, asteroids, moons, and comets-formed is one that planetary scientists are still writing. The tale comes from observations of distant starbirth nebulae and distant planetary systems, studies of the worlds of our own solar system, and computer models that help them understand the data from their observations.02of 06
Start Your Star and Planets with a NebulaThis is a Bok globule, a place where stars begin to form. Hubble Space Telescope/NASA/ESA/STScI
This image is how our solar system looked, some 4.6 billion years ago. Basically, we were a dark nebula-a cloud of gas and dust. Hydrogen gas was here plus heavier elements such as carbon, nitrogen, and silicon, awaiting the right impetus to start forming a star and its planets.
The hydrogen was formed when the universe was born, some 13.7 billion years ago (so our story is REALLY older than we thought). Other elements formed later, inside stars that existed long before our stellar birth cloud began making the Sun. They exploded as supernovae or gasped out their elements as our Sun will do someday.The elements created in in stars became the seeds of future stars and planets. We are part of a grand cosmic recycling experiment.03of 06
It's a Star!A star is born in a cloud of gas and dust, and eventually shines out beyond its stellar cocoon. NASA/ESA/STScI
The gases and dust in the Sun's birth cloud swirled around, influenced by magnetic fields, the actions of passing stars, and possibly the explosion of a nearby supernova. The cloud started to contract, with more material gathering at the center under the influence of gravity. Things heated up, and eventually, the infant Sun was born.
This proto-Sun heated up the clouds of gas and dust and kept gathering in more material. When the temperatures and pressures were high enough, nuclear fusion began in its core. That fuses two atoms of hydrogen together to form an atom of helium, which gives off heat and light, and explains how our Sun and stars work. The image here is a Hubble Space Telescope view of a young stellar object, showing what our Sun may have looked like.04of 06
A Star is Born, Now Let's Build Some Planets!
After the Sun formed, dust, chunks of rock and ice, and clouds of gases made up a huge protoplanetary disk, a region, like those in the Hubble image shown here, where planets form.
The materials in the disk began to stick together to become bigger chunks. The rocky ones built the planets Mercury, Venus, Earth, Mars, and the objects that populate the Asteroid Belt. They were bombarded for the first few billion years of their existence, which further changed them and their surfaces.
The gas giants began as small rocky worlds that attracted hydrogen and helium and lighter elements. These worlds likely formed closer to the Sun and migrated outward to settle into the orbits we see them in today. The icy leftovers populated the Oort Cloud and the Kuiper Belt (where Pluto and most of its sister dwarf planets orbit).
Super-Earth Formation and LossA superEarth forms near its parent star. Did our solar system have some of these? There's evidence to support their existence for a short time in the early solar system. NASA/JPL-Caltech/MIT
Planetary scientists now ask "When did the giant planets form and migrate? What effect did planets have on each other as they formed? What happened to make Venus and Mars the way they are? Did more than one Earth-like planet form?
That last question may have an answer. It turns out that there may have been "super-Earths". They broke up and fell into the baby Sun. What could have caused this?
Baby gas giant Jupiter may be the culprit. It grew incredibly huge. At the same time, the Sun's gravity was tugging on the gas and dust in the disk, which carried the giant Jupiter inward. Young planet Saturn tugged Jupiter the opposite direction, keeping it from disappearing into the Sun. The two planets migrated out and settled into their current orbits.
All that activity wasn't great news for a number of "Super-Earths" that also formed. The motions disrupted their orbits and gravitational influences sent them hurtling into the Sun. The good news is, it also sent planetesimals (the building blocks of planets) into orbit around the Sun, where they eventually formed the inner four planets.
How Can We Know About Long-Gone Worlds?This computer simulation shows the changing orbits of a Jupiter giant in our early solar system (blue), and its effect on the orbits of other planets. K.Batygin/Caltech
How do astronomers know any of this? They observe distant exoplanets and can see these things happening around them. The odd thing is, many of these systems look nothing like our own. They typically have one or more planets much more massive than Earth orbiting closer to their stars than Mercury does to the Sun, but have very few objects at greater distances.
Did our own solar system form differently because of events like the Jupiter-migration event? Astronomers ran computer simulations of planetary formation based on observations around other stars and in our solar system. The result is the Jupiter migration idea. It hasn't been proved yet, but since it is based on actual observations, it's a good first start on understanding just how the planets we have got to be here.