Extrasolar Planets

Extrasolar planets (also known as exoplanets) are defined as planets orbiting stars outside our solar system.  In other words, these are other solar systems beyond our own.  Since the topic is such a hot one in astronomy and astrophysics right now, questions about extrasolar planets can show up in a general education introduction to astronomy, an upper level astrophysics course, a first or second year physics class, and many other places.  Here at 24HourAnswers, we have a team of experts in introductory astronomy, in physics, and in astrophysics ready to help you with your questions about extrasolar planets, or assist you with all types of assignments related to the topic, from essay questions and short answer type questions to detailed calculations.   

The interest in extrasolar planets goes beyond simply the research potential of these planets, but also to future exploration much further down the road.  After all, we are much more familiar with the contents of our own solar system and have yet to find a planet in our own solar system that mimics Earth for its location, its average temperatures, and its ability to sustain life as we know it.  Finding Earth-like planets elsewhere in our galaxy could have profound implications for our future as a civilization.

The History of Extrasolar Planets

The first extrasolar planets were not discovered until 1992 when three extrasolar planets were detected by Aleksander Wolszczan and Dail Frail around a millisecond pulsar.   While this discovery was ground-breaking, the planets detected would hardly be suitable for life, since pulsars are not emitting substantial amounts of radiation in visible wavelengths.  It wasn’t until 1995 that the first extrasolar planet was found around a main sequence star – in this case, 51 Pegasi, a star of similar temperature to our Sun located approximately 50 light years from Earth.  However, the planet that Michel Mayor and Didier Queloz detected around 51 Pegasi is nothing like Earth.  It is extremely close to the star (only 0.05 AU or 5% of the Earth’s distance from the Sun) and weighs half of Jupiter’s weight!  Due to its proximity to its star, scientists estimate temperatures of 1200^oC on the planet.  This type of planet, called a “Hot Jupiter” because its size and proximity to the star it orbits was very common among the first exoplanets discovered.

A lot has changed since 1995.   As of August 5, 2019, there are 4103 confirmed extrasolar planets (exoplanet.eu gives a constantly updated count) with many extrasolar planets found that are the size of the Earth or smaller and located at a wide variety of distances from their host star.  How did we get from 0 in 1991 to over 4000 less than 30 years later?  To answer this question, let’s focus on the way that these extrasolar planets have been detected.  

Detecting and Discovering Extrasolar Planets

Perhaps surprisingly at first, extrasolar planets were not and still are not frequently seen directly in images, even for the closest of extrasolar planets.  The reason for this is two-fold.  The planet and star are often so close together that even with the best telescopes we have, we cannot resolve the separation between the two and the light from the two becomes merged together.  In addition, since the extrasolar planets do not make their own energy but simply reflect the light from the star they orbit, they often do not produce enough light to be detected distinctly from the much brighter star.  Instead, the light from the planet and star is blurred together into one composite point in the sky.  Imagine trying to see a Christmas tree light right next to a giant searchlight while standing 10 miles away – good luck!

Initially, most extrasolar planets were found by what is called the radial velocity method.   What is this method?   First, it is technically incorrect to say that a planet goes around its star or that, for example, the Earth goes directly around the Sun.  In actuality, the star and planet are both moving in orbit around the center of mass of the star and planet system.  This means, for instance, that our Sun is moving in a small orbit around its center of mass.  For example, on average, the Sun travels at about 12 m/s due to the gravitational pull of Jupiter.  To find exoplanets, scientists would look for stars that appeared to be moving in an orbit around a non-visible center of mass.  This orbit can be detected by the Doppler effect.   As the star moves in its orbit, there are times when the star will be coming towards us and times when it is moving away from us.  This motion towards or away from us can be detected as a shift in the wavelength of spectral lines to shorter wavelengths (blueshift) or longer wavelengths (red shift) respectively.  Astronomers have developed elaborate observing strategies to take spectra of hundreds of stars over a night and then look for any changes in the Doppler shift which would then indicate the star was wobbling, possibly due to gravitational influence of an extrasolar planet.  This radial velocity detection method is time and telescope resource expensive, but was the most successful method of detecting extrasolar planets until at least 2009.  

In 2009, extrasolar planet research entered a new age with the launch of the Kepler space telescope.   This space telescope’s sole purpose was to search for extrasolar planets, and it made use of a newer method of extrasolar planet detection, the transit method.  In the transit method, if an extrasolar planet happens to pass directly in front of its star then the overall light coming from the star will dim for a short time as the planet is blocking some of the light that normally comes from the star.  The Kepler space telescope monitored the light coming from approximately 150,000 stars, and as of October 2017 had detected over 2500 confirmed extrasolar planets.  While the initial planets detected in the 1990s and 2000s were primarily large planets close to their stars, this turned out to be a selection effect.  These hot Jupiters had simply been easier to detect.  In comparison, the Kepler mission found extrasolar planets of all sizes with the most commonly found planets being between 1.2 and 3.1 solar masses. 

The Future of Extrasolar Planets

With so many extrasolar planets discovered, the excitement has moved beyond simply finding these exoplanets.  It used to be that the discovery of a single extrasolar planet was front-page news.  Now, announcements of thousands of new extrasolar planets might only receive a brief mention.  Instead, scientists began looking for what they called “Goldilocks planets”.  Goldilocks planets are rocky planets like Earth located in the Habitable Zone of a star.  The Habitable Zone is defined as the area around a star where water could exist on the surface of a planet.  Stars of different temperatures and sizes have different habitable zones, but in all cases habitable zones are fairly narrow.   For example, our Sun’s habitable zone only includes the Earth.  Venus is too close to the Sun and therefore too hot, and Mars is too far away from the Sun and therefore too cold for water to survive long term.  There are now over 30 Goldilocks planets known and the number is increasing.  The next steps up are to see if we can take spectra of these Goldilocks planets to determine whether there is water vapor or methane or other signs of life.  

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