Exoplanets Glossary

The discovery of new planets beyond our Solar System has further expanded on what we thought we knew about what is out there. These distant extrasolar planets or exoplanets have only been confirmed to be in existence within the last twenty years and lots of new discoveries are still being made. This new era of discovery has brought with it new terminology. So here is an A-Z of some terms and associations connected to the discovery of these distant, new worlds.

 

Image 1-Artists impression of an exoplanet in orbit around host star. Credit: NASA.

Image 1-Artists impression of an exoplanet in orbit around host star. Credit: NASA.

 

Alpha Centauri Bb-

The Alpha Centauri star system contains the next closest stars to us after our Sun. Proxima Centauri is located only 4.2 light years away. Supposedly the closest exoplanet to us is also located in this system around one of the other stars, Alpha Centauri B.  This hot, Earth-sized, rocky exoplanet, Alpha Centauri Bb was discovered by astronomers in Geneva last year. However these claims have now been disputed by Artie Hatzes, an American Astronomer working in Germany who was unable to locate this exoplanet. So Alpha Centauri Bb has not been removed from the list of exoplanets but its existence remains currently unverified so it may not after all be the closest exoplanet to the Earth.

b-

Often an exoplanet’s name comes from the name of the host star followed by a lower case letter ‘b’ and subsequent planets discovered then use additional letters of the alphabet. If many planets are discovered at the same time, the planet closest to the star becomes ‘b’ and the others are ordered then by distance to their host star. Often planets can also be named after the instruments used to discovery them, e.g. Kepler 11g. This is the 7th planet discovered orbiting the 11th planet hosting star catalogued by the Kepler spacecraft.

Carbon planets

Carbon is a key component for all life on Earth. However in our inner Solar System oxygen is more prevalent than carbon. Stars which contain a lot of carbon may also have carbon planets in orbit. If high pressures are imposed on these planets this could turn the carbon or graphite into mountains and layers of diamond. The exoplanet, 55 Cancri e has almost 8 times the mass of the Earth with a diameter twice the size and is thought to be a carbon planet. It has temperatures of over 2000°C and a year lasts 18 hours. Carbon planets are not common, with an estimate of only 1 in 1000 being diamond worlds.

 

Could some exoplanets be made of diamonds? Credit:  Wikimedia Commons.

Could some exoplanets be made of diamonds? Credit: Wikimedia Commons.

 

Chthonian planets

This is a class of planets in which the atmosphere is stripped away most likely caused by its close location to its host star. These planets are also extremely dense. As the gases are stripped away a rocky core can be left behind. One of the first chthonian planets discovered was CoRoT-7b.

 

CoRoT mission

The European CoRoT (Convection, Rotation and planetary Transits) satellite was launched at the end of 2006. CoRot was the first mission that successfully detected rocky exoplanets. It also was able to detect ripples coming from the inner structures of stars. These ‘starquakes’ subtly affect the brightness of a star. By using these measurements astronomers can determine the mass and composition of a star and compare with our Sun.  The CoRoT mission was retired in June 2013 after computer failure last year, however the mission did last twice as long as intended, discovered 32 planets and other potentials awaiting confirmation.

Direct imaging

This is a direct method of observing exoplanets. Exoplanets are very faint objects and their light is often washed out by light from neighbouring stars. This method works best for detecting planets which emit infrared light. Most of the planets detected with this method are Jupiter sized or larger and have wider orbits therefore are not shadowed by the light coming from the host star.

 

Directly observed exoplanet HD 95086 b captured by the VLT telescope. Credit: Wikimedia Commons.

Directly observed exoplanet HD 95086 b captured by the VLT telescope. Credit: Wikimedia Commons.

 

Extra-galactic planets

Most of the exoplanets discovered so far are within our Milky Way galaxy. However there are a few planets which have been discovered that are in another galaxy. These are extragalactic planets.

Extra-terrestrial life

The discovery of new worlds has not brought with it new life forms. As far as we know Earth is still the only planet with living beings. Many of the new planets discovered orbit too close to their parent star to support life. So planets which are similar to the Earth with habitable temperatures are where it is thought it is most likely for life to exist. This one is still a waiting game.

 Zombie planet Formalhaut b. This false colour composite image captured by Hubble shows the planets orbit. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute).

Zombie planet Formalhaut b. This false colour composite image captured by Hubble shows the planets orbit. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute).

Fomalhaut b

This is an exoplanet three times the size of Jupiter orbiting the star Fomalhaut. This planet has been directly observed by the Hubble Space Telescope in 2008. However the Spitzer infrared telescope was unable to find it which led to doubts in its existence. However, it was reconfirmed to be there in October 2012. This planet has sometimes been nicknamed the ‘zombie planet’ as it has been revived from nonexistence.

 

 

 

Gaia mission

This European astrometry mission is due to launch  in the next few months. Its mission is to produce a 3D map of our galaxy and survey a billion stars. In the process Gaia may discover new exoplanets.

Habitable Zone

Sometimes also known as the Goldilocks Zone inspired by the story of Goldilocks and the three bears. Earth orbits in the habitable zone, not too close to the Sun or too far away, but just right. It is therefore thought that exoplanets which orbit in a habitable zone, at the right temperature are the most likely to support liquid water and potentially life.

International Astronomical Union (IAU)

The IAU are the governing body that decide how to name and classify celestial objects. With the large number of exoplanets discovered the IAU at the beginning of the year stated they had no plans to name these new worlds. They were destined to be referred to by their official names, that was despite many public competitions and many unofficial names. However, in August they changed their stance and it’s now been announced that popular names will be given to the exoplanets and their moons and the public can have an input. Some of the rules that will have to be considered include, giving the planet just a one word name with 16 characters or less, it must be pronounceable in many languages and the name cannot be used to make revenue. It’s also advised that names and places connected to politics and war are unsuitable. The discoverer will have to agree to the name and when decided it will be publicly be announced by the IAU.

 

Jupiters-Hot

Hot Jupiters are the most common type of exoplanet. They are large gaseous planets similar in size, mass and structure to the largest planet in our Solar System Jupiter. They are also extremely hot due to their close proximity to their parent star. They typically orbit at a distance of 0.15 AU or closer to their star. Mercury the closest planet to the Sun is farther at 0.39AU distant.

Kepler Mission

The Kepler Space Observatory was NASA’s first mission to locate exoplanets the size of the Earth or smaller in or near the habitable zone. It is focused on a fixed location north of the Summer Triangle. The spacecraft was named after the German mathematician Johannes Kepler and was launched in 2009. Kepler so far has discovered 151 confirmed planets with over 3000 potential planets also discovered. However, the spacecraft has had some technical difficulties this year and lost its ability to precisely position itself, and in August it was announced this was unable to be fixed. It is hoped that the data collected will still provide new answers and engineering tests are happening to see what remaining abilities the space craft has.

Artist’s impression of Kepler 62f, one of the many worlds discovered by the Kepler mission. This planet orbits in a habitable zone but its parent star is cooler than our Sun. Credit: NASA Ames/JPL-Caltech

Artist’s impression of Kepler 62f, one of the many worlds discovered by the Kepler mission. This planet orbits in a habitable zone but its parent star is cooler than our Sun. Credit: NASA Ames/JPL-Caltech

 

Light years

A light year is how far light can travel in a year. Light travels at approximately 300 000km/s. A light year therefore is the equivalent to roughly 10 trillion kilometres. The closest exoplanet although unverified is Alpha Centauri Bb at 4.3 light years away.

Moons-Exomoons/Extrasolar Moons

These are natural satellites in orbit around an exoplanet. The gas giants in our Solar System have numerous moons therefore there is likelihood that gas giant exoplanets may also have moons. No moons have been confirmed but some candidates have been discovered by the Kepler spacecraft.

Neptunes- Hot

Hot Neptunes are planets which have a mass and radius similar to Neptune but orbit much closer to their star and are therefore hotter than Neptune.

Image of Neptune - Using what we know about planets in our Solar System can help our knowledge of exoplanets. Credit: NASA.

Image of Neptune – Using what we know about planets in our Solar System can help our knowledge of exoplanets. Credit: NASA.

 

Neptunes-Mini

Mini- Neptunes are exoplanets which are larger than the Earth but smaller than Neptune and orbit Sun-like stars.

Orbital Eccentricity

All planets move in an orbit around their star. Some orbits are circular some are more elliptical. Orbital eccentricity has values from 0-1 and essentially shows how non-circular an orbit is. A perfectly circular orbit has an eccentricity of 0. Values between 0 and 1 then represent an elliptical orbit. The Earth has an eccentricity of 0.017, almost perfectly circular. It is thought that the exoplanets that have short orbits less than 20 days may have an eccentricity close to 0, and those with longer orbital periods may have greater eccentricity.

Parent star

This is the star in which an exoplanet is found to be orbiting, also called host stars. The Sun is the Earth’s parent star.

51 Pegasi b

This was the first exoplanet discovered in orbit around another Sun like star 51Pegasi in 1995. It has unofficially been named Bellerophon after the Greek hero who tamed the winged horse Pegasus, the constellation in which this exoplanet is located.

Image of Bellerophon taming Pegasus. Credit: Wikimedia Commons.

Image of Bellerophon taming Pegasus. Credit: Wikimedia Commons.

 

Qilin

The planet CoRoT-7b was discovered located in the constellation of Monoceros the Unicorn. This is a planet which has a diameter 1.5 times that of the Earth. The unofficial name which has been suggested for this exoplanet is Qilin, named after the mythical Chinese creature similar to a unicorn.

Chinese Qilin, A mythical creature whose Western counterpart is the Unicorn. Credit: Wikimedia Commons.

Chinese Qilin, A mythical creature whose Western counterpart is the Unicorn. Credit: Wikimedia Commons.

 

Radial Velocity

This is a successful ground-based method of discovering exoplanets.  This essentially looks for a wobble of a star which suggests a planet’s gravity is pulling on the star. This is also known as Doppler Spectroscopy.  This method helps in the discovery of larger mass exoplanets but smaller Earth-sized planets would remain undetected with this method.

Super Earth

This term refers to exoplanets which have a mass larger than the Earth’s but less than Uranus or Neptune. The term mini-Neptune is often also used to describe these worlds. A super Earth does not have the same conditions, temperatures or structure as the Earth nor does it orbit in a habitable zone. This term refers only to mass.

Image of Super Earth- These planets may have a similar mass to the Earth but share few other similarities. Credit: NASA.

Image of Super Earth- These planets may have a similar mass to the Earth but share few other similarities. Credit: NASA.

 

Transit method

This method of detecting exoplanets looks for a regular drop in a star’s brightness suggesting a planet is moving or transiting in front of the star. Reoccurrence of the transit allows astronomers to determine orbital length. The reduction of light from the star also helps determine the size and density of the planet.

Unofficial Names

The International Astronomical Union has changed their stance on the naming of exoplanets (see above).  Despite these names not being officially recognised yet some have proved popular. The oldest exoplanet discovered PSR B1620-26 c is nicknamed the Methuselah planet.

VLT images

Most exoplanets are observed indirectly. However the Very Large Telescope (VLT) in Chile has been able to produce a direct viewed image of some exoplanets. In June of this year the VLT imaged an extremely light potential exoplanet HD 95086b.

image of VLT

The search to see exoplanets has been helped by the VLT telecope, to see far beyond our Moon and planets. Credit: Wikimedia.

 

WASP

SuperWASP (Wide Angled Search for Planets) consists of two observatories in South Africa and the Canary Islands and is the UK’s exoplanet search programme. SuperWASP uses 8 wide-angle cameras to survey the skies for any transits. These cameras are not just focused on one part of the sky but survey the entire sky.

Water

The unique feature of our Earth is that water is liquid on the surface. In the hunt for an exoplanet similar to Earth scientists are looking at planets within the habitable zone. It is thought that if an exoplanet has water it may be able to support life.

X-rays

Transits of a planet have been observed in the x-ray wavelength of the spectrum.  When the planet HD 189773b was observed transiting its star the optical light dipped slightly, however the X-rays produced by the star dip 3 or 4 times as much as optical wavelengths. This suggests that this exoplanet’s atmosphere is transparent but is swollen due to the intense radiation it is subjected too. To read more about this planet, see The Blue Planet.

Illustration of the planet HD 189773b transiting across the face of its parent star. Credit: X-ray: NASA/CXC/SAO/K. Poppenhaeger et al; Illustration: NASA/CXC/M. Weiss

Illustration of the planet HD 189773b transiting across the face of its parent star. Credit: X-ray: NASA/CXC/SAO/K. Poppenhaeger et al; Illustration: NASA/CXC/M. Weiss

Year

A year on a planet is the length of time it takes for it to complete one orbit around its parent star. The shortest year in our Solar System is Mercury’s at 88 days and Neptune takes 165 years to orbit the Sun once. In August 2013 it was discovered that the exoplanet Kepler 78b has a year of just less than 9 hours.

Zarmina

This is an unofficial name for the planet Gliese 581g, it has become a well known planet as it located in the habitable zone, not too close or too far from its parent star and is one of the first to be discovered in that area. The discoverer Steven Vogt named this planet after his wife. However its existence has been disputed and it remains an unverified world.

There are many new terms to use in referral to the exoplanets, these are only a selection. Hopefully we will continue to explore the skies in the hunt for faraway, new worlds and learn even more about our Universe.

(Article by Martina Redpath, Education Support Officer)