EXOPLANETS

Image Credit: NASA/JPL-Caltech

PLANETS BEYOND OUR SOLAR SYSTEM

Exoplanets, also called extrasolar planets, are planets that are beyond our own Solar System.  Representing one of today's most exciting areas of astronomy, their study is important because it may one day answer the pivotal question: is life abundant in the universe?

For centuries exoplanets have been theorized to orbit stars other than our Sun.  However the first exoplamet was not confirmed to exist until 1992.  Since then more have been discovered with increasing regularity due to improving detection techniques.  By mid-2013 the total number of confirmed exoplanets was approaching one thousand, these in orbit around approximately seven hundred stars!  Meanwhile over 3,500 more "candidate exoplanets" were awaiting confirmation.  By mid-2019 the existence of over 4,000 exoplanets had been con- firmed!—these orbiting myriads of distant stars.

In addition, at least one exoplanet has been detected which is not in orbit around any star system!  Just 80 light years from Earth, this "free-floating" exoplanet seems to confirm theories that planet-sized bodies can form well away from stars, as well as in protoplanetary disks.  Here is a short ESO video of how this exoplanet may look:  Free-Floating Planet.

How Planets Are Born    from NASA SVS gives a nice video introduction on the formation of planets.  And  Wikipedia's Exoplanet page  is quite informative.

NASA's and JPL's mind-blowing  Exoplanet Exploration,  Exoplanet Archive,  SVS Gallery of Exoplanets  and  Exoplanet Catalog  pages give their latest exoplanet counts and much more.  For an impressive table of related information, here's NASA's  Current List of Confirmed Exoplanets.  And here's a Q & A page that explains  why this count differs from others published.

Want to visit the known exoplanets?  You can streak across the cosmos to tour them in 3-D with NASA's excellent  Eyes on Exoplanets, an awesome interactive feature.  Here are the links to its tutorial video pages:  Tutorial 1  Tutorial 2  Tutorial 3.

And here are two awesome interactives that let you examine the sizes and orbits of exoplanets:  Goldilocks  and  288 Exoplanets  .

FINDING EXOPLANETS

Image Credit: NASA/JPL-Caltech

Astronomers have devised clever methods of detecting exoplanets.  Most involve measuring subtle, exoplanet-induced variations in the electromagnetic radiation that reaches us from the stars they orbit.  These variations are generally quite small and may manifest themselves in the radiation's direction, intensity, spectrum, timing, consistency and more.

One of the easiest-to-understand ways of detecting exoplanets is the "transit method", which measures the tiny decrease in light reaching us from a star when one or more of its planets pass in front of it.  Kepler Transit Graph

Another of the easiest-to-understand ways of detecting exoplanets is the "astrometry method", also often called the "wiggle method".  This accurately measures the position of a star as it moves through space.  If the star has one or more planets, their mutual gravity may cause the star's path to "wiggle" or oscillate like the star in the animation directly above.

You'll find information on the ways astronomers find planets in orbit around other stars in NASA's  5 Ways to Find a Planet  and on Wikipedia's   Methods of Detecting Exoplanets  pages.

PROPERTIES OF EXOPLANETS

PHYSICAL PROPERTIES

As the NASA/Ames/JPL-Caltech diagram above sug- gests, exoplants are proving quite varied.  In fact, they are proving to be more varied than the planets in our own Solar System!  Given the variety of stars they orbit, and the diverse conditions possible around any star and in collapsing interstellar molecular clouds, this is not surprising.  Scientists speculate that the range of possible types of exoplanets is vast indeed.

Image Credit: Marc Kuchner/NASA GSFC

Many of the earliest confirmed exoplanets were found using methods that favor detecting sizeable planets in modest-sized, short-period orbits (some as short as a few days!)  This is why so many have been dubbed "hot Jupiters"!  Even so, considerable numbers of smaller, Earth-sized exoplanets have now been detected, and over a billion are now estimated to exist in our galaxy alone.  Considering all types, exoplanets in our galaxy may total in the hundreds of billions, perhaps trillions!  And that estimate seems to continue to rise!

Image Credit: NASA - Kepler Mission

Of particular interest in the quest to learn more about exoplanets is the treasure trove of secrets that their atmospheres may one day reveal.  Its atmosphere can give hints as to whether an exoplanet is likely to harbor conditions that may support life.  (To this end, Hubble spied the first atmosphere of an exoplanet in the year 2000!)  Of course, scientists have long recognized that life found on exoplanets may exist under conditions radically different from those found here on Earth.

Here are links to some short but cool NASA videos that show some of the types of exoplanets we are already discovering or are one day likely to encounter.  Viewed in full-screen they look awesome!
      An Exoplanet Most Like Earth  
      Earth-like World
      Water World
      Kuiper Belt World
      Hot Jupiter
      Kepler 10-b
      Weird Warm Spot on Exoplanet  
      Kepler-20, An Unusual Planetary System
      Hot Lava World  
      HD 189733b Exoplanet Animation  
      Kepler: Diversity of life possible on other planets

ORBITAL PROPERTIES

As mentioned above, many of the earliest confirmed exoplanets were found using methods that favor detecting sizeable planets in modest-sized, short-period orbits (some as short as a few days!)  However, other techniques have identified exoplanets that orbit considerably further from their host stars.

Here's a link to another interactive that can help you get a feel for the orbital properties of exoplanets, a NY Times 2-D animated diagram,  Kepler's Tally of Planets  that shows the relative numbers and sizes of the exoplanetary systems discovered by Kepler.

HABITABLE ZONES

Image Credit: NASA/Ames/JPL-Caltech

One of the most important aspects of a stellar system which possesses exoplanets is its "habitable zone".  Because our quest to understand exoplanets is largely a quest to find extraterrestrial life, and because life as we know it requires liquid water to exist, a star system's habitable zone is the region around its star (or stars) where a planet-sized body could be expected to maintain liquid water on its surface.  This has also been popularly called the "Goldilocks zone", because it is the region where it is neither too hot, nor too cold, for liquid water to exist, but rather is "just right".

The sizes of habitable zones depend on several factors, including the sizes, ages and colors of the stars which they envelop.  Now you can study various aspects of habitable zones with  Circumstellar Habitable Zone Simulator

As this short NASA video shows, the temperature and size of a star determine the size of its habitable zone:  Comparative Life Zones.  Excitement in the astronomical community is growing as more and more Earth-sized and Earth-like planets are being found in their host stars'  habitable zones.