Sunday, April 28, 2013

11 awesome Exoplanet facts

The first exoplanet was discovered in 1992. This discovery was unexpected in many ways, first the
planet is in orbit around a pulsar, a highly magnetized stellar remnant from a supernova event, but also because there were not looking for it. The search for exoplanets had been actively investigated since the late 70’s but no one was looking around dead stars. It was not until 1995 that the first exoplanet was discovered around an ordinary main sequence star and that presented some surprises of its own.

The first exoplanets discovered were labelled ‘Hot Jupiters’. These planets, although similar in radius to Jupiter, orbit their stars so close that they are tidally locked in place with one side in permanent daylight and the other in perpetual darkness. The close proximity to their star means it can get incredibly hot. The hottest thus far is WASP-12b with a dayside temperature of around 2,500°C that is hotter than some stars*. This orbital distance also means that a year on these worlds is just a matter of days allowing us to take multiple measurements of the planet and the star over short periods of time. 

Since 1992 over 870 exoplanets have been discovered. As of today, April 28th 2013, there are 872 planets in 683 stellar systems ( That is an average of 41.5 new planets
discovered every year for the past 21 years, and that is just in our neighbourhood within a tiny portion of our galaxy. 

There are potentially hundreds of billions of planets in our galaxy. Thanks to surveys like Kepler, WASP and HAT (just to name a few); we can work out how many planets there could be orbiting other stars that we cannot yet detect.  And it comes to a mind-boggling number considering just over 20 years ago our solar system seemed like an anomaly but it is not an easy thing to do.

It’s like looking for a flee passing in front of a street lamp over 1km away. Each of the surveys I mentioned above look for the planet as it transits its star. This means they monitor the starlight over a long period of time and look for characteristic dips in the light, which indicate that there is something periodically blocking out a small part of the star. From this we can determine the radius relative to the star, with the frequency of the transit giving us the orbital period.  

The closest exoplanet to us is Alpha Centauri Bb. Although it orbits the Suns closest star at just over 4 light years away it was not discovered until 2012 as it has such a small influence on its star. Alpha Cent Bb was discovered using radial velocity measurements (link to other blog post) and from this we can calculate its minimum mass to be just slightly higher than that of the Earth and 96% closer to its star. So it is not a potential holiday destination and even with current technology it would take us  years to get there.

The habitable zone of different stars
Just 10 worlds have been discovered where liquid water could exist on their surface. The Planetary habitability laboratory, from the University of Puerto Rico at Arecibo, has been cataloguing those worlds that lie in what is popularly called the goldilocks zone. The region around a star where the temperature is just right for liquid water to be able to survive on the surface, assuming the planet has a ‘surface’ as we know it, and its atmosphere is just right to sustain its presence. We can get some of this information by working out the bulk density of the planet.

The bulk density of exoplanets ranges from Styrofoam to solid lead. If we can measure the minimum mass of the planet through radial velocity, and the radius of the planet through its transit, we can calculate the bulk density of the planet. This allows us to draw some conclusions as to its likely composition; however, we are still just making educated guesses at this point as materials will act differently as you change the conditions. The Earth’s bulk density for instance is 5.5 g/cm3, somewhere between Iron and silicon, which would lead us to conclude that it is a rocky world.   

No exoplanet is exactly like another. Like our solar system there is a wonderful and surprising diversity of different worlds out there. From their radius, mass, and density; to what we have been able to detect in their atmospheres. We have even detected a planet losing some of its atmosphere due to extreme stellar activity**, and planets orbiting multiple stars***. The discovery of exoplanets has really tested each of our imaginations about what is possible. 

Multiple planet systems
Our solar system still holds the record for total number of planets. There are 130 multiple planet systems found so far. The largest have six planets; Kepler-11 (b,c,d,e,f,g), HD 10180 (c,d,e,f,g,h), HD 40307 (b,c,d,e,f,g). And these are just the planets we can detect, with only 20 years of practice and technology development behind us. Imagine what other worlds these systems might hold that we cannot yet see, or any other systems and stars for that matter. 

For your bonus 11th fact.

Thor - Supreme commander of the Asgard fleet
Aliens are out there. More than just letting the statistics speak for itself, we have found some startling evidence on our own planet to suggest that life is not impossible but probable. New studies are popping up everywhere into astrobiology and the potential for life in the most extreme of environments.

The possibilities appear to be endless. 


*Red star range in temperature between roughly 800-3500°C
**HD 189733b had a portion of its atmosphere blown off due to an x-ray stellar flare event that was monitored using ESA's XMM-Newton X-ray space telescope, while a transit of the planet was observed in the visual with NASA’s Hubble Space Telescope, showing a large atmospheric tail in the planets wake. (
*** Planet Hunters: Transiting circumbinary planet in a quadruple star system (

Also check out the exoplanet databases:
and the habitable exoplanet database,

There is also this great interactive exoplanet catalog based on the XKCD comic ( where you can find out some things about the diffrent planets. -
All the kepler planets detected from astronomy-to-zoology

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