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The Milky Way is home to hundreds of billions of planets, an estimate based on the thousands of known worlds discovered just within the last few decades. With this much information, astronomers work to understand the similarities and differences between planetary systems, including our Solar System. This field encompasses research on the planets, comets, and other inhabitants of the Solar System, as well as studies of exoplanets and newborn planetary systems.

Our Work

Center for Astrophysics | Harvard & Smithsonian scientists engage in many types of planetary systems research:

  • Finding and describing new exoplanets using TESS and other observatories. TESS is designed to observe stars closer to the Solar System than those studied by other observatories, providing targets for other telescopes for follow-up observations.
    NASA Prepares to Launch Next Mission to Search the Sky for New Worlds

  • Hunting for potentially habitable exoplanets using robotic observatories. Those include the CfA’s MEarth Project, which focuses on planets orbiting red dwarf stars, and the MINiature Exoplanet Radial Velocity Array (MINERVA), which locates planets orbiting Sun-like stars.
    MINERVA: Hunting for Earth's Twin

  • Identifying the chemical composition of newborn star systems, and comparing that to the Solar System. Using the Atacama Large Millimeter/submillimeter Array (ALMA), the CfA’s Submillimeter Array (SMA), and other observatories, astronomers have identified complex organic molecules in the regions around newborn stars. Similar chemicals have been found on comets, which are fragments left over from the era when the Solar System first formed.
    Astronomers Discover Traces of Methyl Chloride around Infant Stars and Nearby Comet

  • Finding infant planets that might be Earth-like. Exoplanets orbiting stars in Earth-like orbits are difficult to detect, but ALMA and other observatories are capable of identifying newborn worlds at ever-closer orbits around their host stars. Studying how potentially habitable planets form helps us understand our own origins, and how common Earth-like worlds might be in the galaxy.
    A Planet Is Forming in an Earth-like Orbit around a Young Star

Number of confirmed exoplanets

The Diversity of Planets

The Solar System contains four rocky planets, two large gaseous planets, and two other giant worlds, along with five dwarf planets and a wealth of moons, comets, asteroids, and icy worlds. The challenges of observing other star systems means we mostly know about planets orbiting close in to their host stars, with very little information so far about planets orbiting farther out — much less moons, asteroids, and so on. However, a combination of theory and observation is bringing us to a fuller picture of the possible planetary systems, what they contain, and how they were formed.

  • In the early years of exoplanet research, astronomers were happy just to discover any planet. Today, the focus is on classifying all the systems discovered so far, and hunting for smaller planets orbiting farther out from their host stars. In particular, researchers want to find Earth-sized planets in the habitable zones of their stars: the range of distances where liquid water could conceivably exist. Next-generation observatories such as NASA's Transiting Exoplanet Survey Satellite (TESS) are designed for that purpose.

  • Many identified exoplanets are different from what we see in the Solar System. Planets more massive than Jupiter are common, but most exoplanets fall between Earth and Neptune in size or mass. These “super-Earths” are probably rocky, which raises questions about how they might differ from the inner planets of the Solar System and whether they could support life as we understand it.

  • Until the exoplanet revolution, our understanding of planet formation was based entirely on the Solar System. With the extra information from other planetary systems, researchers have a clearer picture of the complexities of how planets form and migrate in infant star systems. Researchers combine theoretical simulations with observations of newborn planetary systems to understand how the diverse worlds we see came into existence. That includes the distribution of atoms and molecules making planets, particularly those required for Earth-like life.

  • Of course, the best understood planetary system is our own Solar System. Comets and asteroids are remnants of the early years of the Solar System’s existence, providing us with a look at the environment before Earth formed. Astronomers have found similar chemical signatures on comets and in distant star systems, indicating some common processes.

  • Our best observations of atmospheres also come from planets in our Solar System, including the planet we know best: Earth. Understanding the differences between planets in our Solar System allows us to create models for exoplanet atmospheres. In addition, researchers are developing new methods to detect molecules in the atmospheres of distant worlds, particularly those like water or oxygen that are closely linked to life on Earth.