The Search for Exoplanets: What Astronomers Know

There's no place like home ... unless we can find another home in a galaxy far, far away. Let's see how that is working out for us.


Join the search for another Earth in this 24-lecture series that explores the brand-new science of exoplanets-worlds that exist outside of our solar system.


Joshua N. Winn
Joshua N. Winn

There are so many reasons to study exoplanets, including exploration, the search for life, the rich physics problem of planet formation, and the technological challenge.


Massachusetts Institute of Technology
Dr. Joshua N. Winn is the Professor of Astrophysical Sciences at Princeton University. After earning his Ph.D. in Physics from MIT, he held fellowships from the National Science Foundation and NASA at the Harvard-Smithsonian Center for Astrophysics. Dr. Winn's research goals are to explore the properties of planets around other stars, understand how planets form and evolve, and make progress on the age-old question of whether there are other planets capable of supporting life. He was a member of the science team of NASA's Kepler mission and is the Deputy Science Director of a future NASA mission called the Transiting Exoplanet Survey Satellite. He has authored or coauthored more than 100 scientific articles on the subject of exoplanetary science. At MIT, Dr. Winn teaches physics and astronomy and has won several awards for his dedication to his students, including the Buechner Faculty Teaching Prize in 2008 and the School of Science Prize for Excellence in Graduate Teaching in 2013. His talent for communicating science to the general public was honed during graduate school, when he wrote for the science section of The Economist.

By This Professor

The Search for Exoplanets: What Astronomers Know


Why Study Exoplanets?

01: Why Study Exoplanets?

Learn about the exciting mission of exoplanetary science-the study of planets orbiting stars beyond the Sun. Review the eight planets in our solar system, which provide a baseline for understanding the more than 1,000 worlds recently discovered in our region of the Milky Way galaxy....

31 min
How to Find an Exoplanet

02: How to Find an Exoplanet

Given the extreme faintness of a planet relative to the star it orbits, how can astronomers possibly find it? Learn about direct and indirect methods of detection. As an example of the indirect method, discover why a planet causes a star's position to change, providing a strategy for locating exoplanets without seeing them....

30 min
Doppler and Transit Planet-Finding Methods

03: Doppler and Transit Planet-Finding Methods

Explore two other indirect approaches for finding exoplanets: first, by measuring the Doppler shift in the color of a star due to the pull of an unseen orbiting planet; and second, by measuring the tiny drop in the brightness of a star as a planet transits in front of it....

32 min
Pioneers of Planet Searching

04: Pioneers of Planet Searching

Chart the history of exoplanet hunting-from a famous false signal in the 1960s, through ambiguous discoveries in the 1980s, to the big breakthrough in the 1990s, when dozens of exoplanets turned up. Astronomers were stunned to find planets unlike anything in the solar system....

31 min
The Misplaced Giant Planets

05: The Misplaced Giant Planets

Investigate 51 Pegasi b, the first planet detected around a Sun-like star, which shocked astronomers by being roughly the size of Jupiter but in an orbit much closer to its star than Mercury is to the Sun. Probe the strange characteristics of these "hot Jupiters," which have turned up around many stars....

30 min
Explaining the Misplaced Giant Planets

06: Explaining the Misplaced Giant Planets

The standard theory of planet formation is based on our solar system. But does this view require revision based on the existence of misplaced giant planets-hot Jupiters circling close to their parent stars? Compare competing theories that try to resolve this conflict....

31 min
The Transits of Exoplanets

07: The Transits of Exoplanets

A tiny percentage of exoplanets can be detected transiting-or passing in front of-their host stars. Combined with Doppler shifts, transits provide information about a planet's size, mass, density, and likely composition. Learn how ambitious amateur astronomers can use this detection technique in their own backyards....

32 min
Sniffing Planetary Atmospheres

08: Sniffing Planetary Atmospheres

Survey the history of spectroscopy to understand how a telescope and a diffraction grating can disclose the composition of a star and its planet. Then learn how transits and occultations are ideal for analyzing planetary atmospheres, paving the way for the search for signatures of life....

31 min
Stellar Rotation and Planetary Revolution

09: Stellar Rotation and Planetary Revolution

Trace Professor Winn's own search for the subtle signs that tell whether a star has a tilted axis. Discover why this is an important clue in the mystery of misplaced giant planets. Also hear how he chanced into the field of exoplanetary science....

30 min
Super-Earths or Mini-Neptunes?

10: Super-Earths or Mini-Neptunes?

Learn how a sensitive new instrument led the way in finding planets smaller than the Jupiter-sized giants that dominated the earliest exoplanetary discoveries. Halfway in size between Earth and Neptune, these worlds have uncertain properties. For clues about their nature, consider how our solar system formed....

31 min
Transiting Planets and the Kepler Mission

11: Transiting Planets and the Kepler Mission

The planet search took a giant leap forward in 2009 with the launch of the Kepler spacecraft, which used the transit technique to observe nearly 200,000 stars over a four-year period. Study Kepler's goals, results, and the persistence of the astronomer who championed it....

31 min
Compact Multiplanet Systems

12: Compact Multiplanet Systems

Dig deeper into the treasure trove of data from the Kepler mission, which discovered hundreds of compact multiplanet systems, with planets much more closely packed than in our solar system. Explore the dynamics of these groupings, which have planets interacting strongly through mutual gravitation....

31 min
Planets Circling Two Stars

13: Planets Circling Two Stars

See how data from the Kepler spacecraft confirms a scenario straight out of the movie Star Wars: a planet with two suns. Investigate the tricky orbital mechanics of these systems. A double star also complicates the heating and cooling cycle on a planet. However, the view is spectacular!...

31 min
Lava Worlds

14: Lava Worlds

Explore the theoretical limit of the smallest possible orbit for a planet, taking into consideration tidal stresses and other destructive processes. Then focus on Professor Winn's search for such objects, which found probable lava worlds-planets heated to rock-melting temperatures by their extreme closeness to their host stars....

31 min
Earthlike Planets

15: Earthlike Planets

Begin your search for planets that may harbor life by studying the conditions that make Earth habitable, including its distance from the Sun, surface temperature, atmosphere, and oceans. Then examine strategies for finding earthlike planets and the progress to date....

31 min
Living with a Dwarf Star

16: Living with a Dwarf Star

The most common stars are class M dwarf stars, which are smaller and less luminous than the Sun (class G). Earth-sized planets are much easier to detect around M-dwarf stars, especially if the planets are within the relatively close-in habitable zone. Explore examples and the prospect for life on such worlds....

31 min
Living with a Giant Star

17: Living with a Giant Star

In billions of years, the Sun will expand into a red giant, possibly engulfing Earth. Learn how planet-finding techniques give astronomers insight into the processes inside giant stars. Then study the planets around these behemoths for clues about Earth's ultimate fate....

31 min
Our Nearest Exoplanetary Neighbors

18: Our Nearest Exoplanetary Neighbors

Pinpoint the location of the nearest exoplanetary systems to Earth. First, get the big picture on the layout of our Milky Way galaxy, its size, and the Sun's position. Also learn why the Kepler spacecraft focused on exoplanets much more distant than those targeted by the Doppler technique....

30 min
Finding Planets with Gravitational Lensing

19: Finding Planets with Gravitational Lensing

Get a lesson in Einstein's general theory of relativity to understand an effect called gravitational microlensing, which allows astronomers to deduce a planet's existence without recording any light from the planet or its host star. This technique reveals exoplanets that would otherwise go undetected....

31 min
Finding Planets with Direct Imaging

20: Finding Planets with Direct Imaging

Turn to the most obvious way to find exoplanets: direct imaging. Explore the optics of telescopes to learn why spotting an exoplanet next to its parent star is so difficult. Then see how this limitation has been overcome in a handful of cases....

30 min
Near-Term Future Planet-Finding Projects

21: Near-Term Future Planet-Finding Projects

The success of exoplanetary science has spurred a wave of new projects to increase our knowledge of worlds beyond our solar system. Survey ground- and space-based programs that are now in the works. Professor Winn gives a preview of a space mission that he and his MIT colleagues are designing....

28 min
Long-Term Future Planet-Finding Projects

22: Long-Term Future Planet-Finding Projects

Peer into the future at ambitious projects that may one day succeed in collecting light directly from an Earth-sized planet in the habitable zone of a nearby star. Examine three different engineering approaches: the coronagraph, interferometer, and starshade....

30 min
The Search for Life on Exoplanets

23: The Search for Life on Exoplanets

Join the quest for life on exoplanets, focusing on the search for extraterrestrial intelligence (SETI)-a hunt for signals from alien civilizations inspired by a landmark paper in 1959. See how the famous Drake equation points to factors that determine how many such civilizations may exist....

31 min
Coming Soon: Biosignatures, Moons, and More!

24: Coming Soon: Biosignatures, Moons, and More!

Explore the distinctive biosignatures that show the presence of life of any kind on an exoplanet. Then close with Professor Winn's tip sheet on exoplanetary discoveries likely in the near future-from evidence of moons to planets being destroyed by giant stars....

31 min