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The Milky Way alone probably contains hundreds of billions of planets, based on the thousands of exoplanets we’ve already identified. These planets share a history and origin with their host stars, and none of the star systems observed so far resemble the Solar System. Modern studies of planet formation include comparing exoplanetary systems, identification of protoplanetary disks around newborn stars, and computer models to trace the creation of planets from their origins in interstellar dust and gas.

Our Work

Center for Astrophysics | Harvard & Smithsonian astronomers study the formation of planets:

  • Looking for complex organic molecules in protoplanetary disks. Astronomers use the Atacama Large Millimeter/submillimeter Array (ALMA), the CfA’s Submillimeter Array (SMA), and other instruments capable of identifying light absorbed by these molecules. Knowing how these materials are distributed in a protoplanetary disk helps us determine how life could arise on planets.
    Complex Organic Molecules Discovered in Infant Star System

  • Searching for newborn planets in protoplanetary disks. Protoplanets gather material onto themselves from the protoplanetary disk, creating a gap. The position and size of these gaps can reveal if the new planets are rocky or gaseous, how massive they are tell us, and whether they migrate from their initial positions. However, not every gap contains a planet, so careful observation is needed to determine what’s going on.
    Disk Gaps Don't Always Signal Planets

  • Searching also for Earth-like planets in infant star systems. While exoplanet hunts mostly find planets close in to their stars, studies of protoplanets are more likely to identify planets farther out. That’s helpful if we want to find planets like ours, which orbits in the “habitable zone” where liquid water can potentially exist on the planet’s surface.
    A Planet Is Forming in an Earth-like Orbit around a Young Star

  • Modeling planet formation to infer exoplanet structure and composition. Exoplanets are too small for us to take images of them, much less study details of their interiors. However, theoretical models fill in that knowledge, using data from real planetary systems. That helps us understand “super-Earths”, a common type of rocky world larger than Earth that doesn’t exist in the Solar System.
    Earth-like Planets Have Earth-like Interiors

  • Studying the chemical composition of comets to understand planet formation. Comets are relics of the protoplanetary disk that gave birth to the Solar System. Astronomers compare the molecules observed in protoplanetary to those on the surface of comets to understand the primordial chemistry of newborn star systems.
    Astronomers Discover Traces of Methyl Chloride around Infant Stars and Nearby Comet

  • Participating in space probe missions to asteroids. Like comets, asteroids are leftovers from the formation of the Solar System, carrying chemical traces of the protoplanetary disk. The Origins, Spectral Interpretation, Resource Identification, Security, Regolith, Explorer (OSIRIS-REx) is designed to study the near-Earth asteroid Bennu and return a sample of its surface material to Earth.
    Asteroid Mission Will Carry Student X-ray Experiment

Protoplanetary Disks

Stars form from cold interstellar molecular clouds. As they collapse into protostars under the force of gravity, the remaining matter forms a spinning disk. Eventually the star stops accreting matter, leaving the disk in orbit around it. The leftover gas and dust inside that protoplanetary disk become the ingredients for planet formation.

For that reason, the chemical composition of the protoplanetary disk determines the composition of the eventual planets that form from it. Since the newborn star and its planets had a common origin in the cloud that made them, their history and composition are linked. Organic molecules present in the original molecular cloud become part of the protoplanetary disk and planets that form from it.

Astronomers have identified a large number of protoplanetary disks, including some with gaps that might reveal the presence of a planet being formed. The structure of these disks provides clues to where planets form, and whether they change orbits after formation.  

comparison illustration of the interiors of Earth and the exoplanet Kepler-93b

This artist's illustration compares the interior structures of Earth (left) with the exoplanet Kepler-93b (right). Even though the exoplanet is four times Earth's mass, research shows rocky planets all likely have the same internal structure.

Credit: M. Weiss/CfA

Learning From Exoplanets and Their Host Stars

None of the thousands of exoplanetary systems discovered so far resemble the Solar System. While this is mostly because of the way we detect exoplanets, our understanding of planet formation has to account for the diversity of worlds we observe. Many exoplanets orbit closer to their host star than Mercury orbits the Sun, and quite a few star systems contain giant planets in the inner part of the system. In addition, several systems have multiple planets in orbits that are much closer together than any two planets in the Solar System.

Additionally, many known exoplanets orbit red dwarf stars, which are much less hot than the Sun. A number of these stars are also very active, producing violent flares that would likely affect the planet. Some other stars may have been too active during their early life to allow planets to form.

However, even the best observations of protoplanets and exoplanets aren’t good enough to reveal exactly how planets form and migrate. For that reason, astronomers develop computer simulations to understand how the protoplanetary disk produces the types of planets we observe, and how atoms and molecules are distributed to create the kinds of worlds we observe in the Solar System and beyond.