Exoplanet Catalog Glossary

Below we provide a simple explanation of all the terms found throughout the Exoplanet Catalog. These definitions provide you with all you need to know to navigate and understand the catalog.

To learn more about the exoplanet animations, go to the Explanation of Exoplanet Animations.

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Alternative Star Name: Another name of the star. Some stars have multiple well-known names.

Argument of Periapse: An orbital angle describing the orientation of the orbit with respect to Earth. This angle only has meaning in eccentric orbits: if the shortest part of the orbit is in the same direction as the Earth, then the argument of periapse is 90º.

Comments: Lists any additional comments on the exoplanet or parent star.

Discovery Code: Shows how and when the exoplanet was discovered. The first two letters signify the method of discovery: RV = Radial Velocity method, TR = photometric TRansit method, DI = Direct Imaging, AS=AStrometry, ML = MicroLensing, TI=TIming, or OT=OTher. The final two numbers signify the last two digits of the year of discovery (defined as the year the exoplanet was announced or published). For example, 51 Pegasi b was discovered by the Radial Velocity technique in 1995, so its discovery code is RV95. Most exoplanets have observational data stretching for months or years before and after the year of discovery and some have been investigated by multiple methods.

Eccentricity: A number between 0 and 1 that describes the shape of an orbit. An ellipse with zero eccentricity is just a circle, while ellipses with eccentricities near one are very stretched out. In the case of exoplanetary orbits, eccentricities smaller than about 0.1 are difficult to measure and so planets are sometimes assumed to have zero eccentricity.

Inclination: An angle that describes the orientation of the orbit with respect to the Earth. An orbit with an inclination of 0º is called “face-on” since the orbit is perpendicular to the line of sight and orbits in the plane of the sky. An orbit with an inclination of 90º is called “edge-on” since the orbit is parallel to the line of sight. The planet will transit its parent star (passing between the star and the Earth) only when the orbital inclination is near 90º. Furthermore, the radial velocity signal measured by Doppler spectroscopy is maximized when the inclination is near 90º, because the motion of the star is then primarily along the line-of-sight. Generally the orbital inclination is not known, which leads to an uncertainty in the measurement of planetary masses.

Metal Enhancement Factor: The amount of “metals” in the star as compared to the Sun. Using a prism to spread out starlight into its constituent colors, scientists can measure the composition of stellar atmospheres. For astronomers, “metals” refers to all elements heavier than Hydrogen and Helium (including elements we usually call gases, such as Oxygen). The metal enhancement factor is the ratio of the metals in the star as compared to the Sun. For example, if this factor is 3, then the “metal” concentration in the star is three times greater than in our own Sun (about 1.7%). The abundance of metals in stars is usually called the “metallicity”, which is the base 10 logarithm of the metal enhancement factor. This property of the star is very interesting to planet-hunters because of a strong correlation between metal enhancement and the presence of (detectable) exoplanets. In other words, metal-enriched stars are much more likely to have exoplanets. It appears that this correlation can be explained by noting that stars with high concentrations of metals originally had more planet-building material and thus were much more likely to produce detectable exoplanets.

Number of Planets in System: The number of known exoplanets orbiting this star. Also known as the multiplicity, this number measures how many exoplanets are known orbiting the star in question. Currently, the system with the highest multiplicity is 55 Cancri with 5 planets. Most stars certainly have additional unknown planets that have yet to be discovered.

Orbital Period: The time taken in days for the exoplanet to make a complete revolution around its parent star. Though the Earth takes 1 year to make a complete revolution, some exoplanets zip around their stars in less than a day!

Planet Density: The density of the planet (mass divided by volume), measured in grams per cubic centimeter. Densities are useful for determining the bulk composition of exoplanets. For example, gas giants tend to have densities near 1 g/cc, while rocky terrestrial planets have densities near 5 g/cc. The density of an exoplanet can only be calculated if the mass and radius are both known.

Planet Mass: The mass of the planet, measured in Jupiter masses. Most exoplanets are gas giants, like Jupiter, and so Jupiter’s mass of 1.9 x 10²⁷ kg (2.1 x 10²⁴ tons) is a natural metric. To convert Jupiter masses to Earth masses multiply by 317.8. The radial velocity technique only reveals a “minimum mass” of the planet, which is the true mass times the trigonometric sine of the inclination angle. Unless some other technique is available, the inclination, and therefore true mass, remains unknown. When only the minimum mass is known, the catalog entry for the mass is italicized as a reminder that these planets may be more massive than we expect.

Planet Name: The name given to the exoplanet, often the star name with an additional lower-case letter. In systems with multiple planets, the letters go in order of the discovery, e.g. 55 Cancri b was discovered before 55 Cancri c.

Planet Radius: The size of the planet (half the diameter), measured in Jupiter radii. Most exoplanets are gas giants, like Jupiter, and so Jupiter’s radius of 71,500 km (44,400 miles) is often used to measure exoplanet radii. To convert to Earth radii (6,378 km or 3,963 miles), multiply the radius by 11.2. Most planets do not have measured radii, unless they are transiting planets; if the radius is not known then it is not listed. Because of the way materials are compressed in planets, even exoplanets much more massive than Jupiter are not much larger in radius.

Right Ascension and Declination: Coordinates that describe the approximate location of the star in the sky. These are similar to longitude and latitude on the surface of the Earth. Using these celestial coordinates, amateurs and professional scientists can locate the exact positions of stars in the sky. If the declination is positive (or unsigned), then the star is best seen from the Earth’s Northern hemisphere. Negative declinations denote stars most easily seen from the Earth’s Southern hemisphere.

Semi-major Axis: A measure of the size of an exoplanetary orbit. In the early 1600’s Johannes Kepler discovered that planets always moved in elliptical orbits. The longest distance across an ellipse is called the major axis (the shortest distance across is called the minor axis); the semi-major axis is half the longest distance across an ellipse. If an ellipse has zero eccentricity, then it is a circle and the semi-major axis is just the radius of the circle. The semi-major axis is measured in AU or Astronomical Units, where 1 AU is the semi-major axis of the Earth’s orbit around the Sun.

Spectral Type: The astronomical spectral classification of the host star. In the astronomical community, stars are categorized according to “spectral type”, which is useful for distinguishing between large blue O, B, and A stars, solar-like F, G, and K stars, and small red M stars. Numbers ranging from 0 to 9 are also added as sub-categories: the Sun is a G2V star. Finally, a roman numeral is added to designate the evolutionary phase of the star: stars start out as typical “main sequence” dwarfs (V) and can evolve near the end of their lifetimes to become subgiant (III) or giant (I) stars. The mass, radius, spectral type, and temperature of a star are often related with more massive, larger, early-type stars having high temperatures and less massive, smaller, later-type stars having lower temperatures.

Star Mass: The mass of the exoplanet host star, in solar masses. Most stars have masses similar to our Sun’s mass (1.9 x 10³⁰ kg or 2.2 x 10²⁷ tons). The mass, radius, spectral type, and temperature of a star are often related with more massive, larger, early-type stars having high temperatures and less massive, smaller, later-type stars having lower temperatures. Typically, astronomical observations can not directly measure the stellar mass, which is often inferred from other measurements and stellar models.

Star Name: The astronomical name of the star. The brightest stars (e.g. Fomalhaut) have names that date to ancient times, while moderately bright stars typically have catalog names: the majority of planet-hosting stars were characterized in the Henry-Draper catalog and bear the initials HD. Most stars have a wide variety of names corresponding to different catalogs and observers. Many transit surveys (e.g. TrES, OGLE, HAT, and WASP) gave easy-to-remember names to the stars hosting transiting planets.

Star Radius: The size of the exoplanet host star in solar radii. Most stars have radii similar to our Sun’s radius (695,500 Km or about 432,000 miles). The mass, radius, spectral type, and temperature of a star are often related with more massive, larger, early-type stars having high temperatures and less massive, smaller, later-type stars having lower temperatures. Typically, astronomical observations can not directly measure the stellar radius, which is often inferred from other measurements and stellar models.

Stellar Brightness: The brightness of the star as seen from Earth in V magnitudes. The magnitude system describes how bright a star appears in the sky or through a telescope, with smaller numbers corresponding to brighter stars. First magnitude stars (V=1) can be seen even in light-polluted cities, but dark nights and sharp eyes are needed to detect the faintest naked-eye stars with V=6. Since more distant objects appear fainter, the brightest stars are also usually the closest. Exoplanets are usually found around bright stars as the majority of exoplanet hunting techniques benefit from having bright and/or nearby stars.

Stellar Temperature: The estimated surface temperature of the host star in Kelvin. The Sun’s surface temperature is about 5,800 K, but stellar temperatures can range from more than 40,000 K to less than 3,000 K. The temperature of a star is generally directly related to the amount of energy it gives off (called luminosity). Therefore, planets around stars with higher stellar temperatures are themselves hotter, all else being equal. The mass, radius, spectral type, and temperature of a star are often related with more massive, larger, early-type stars having high temperatures and less massive, smaller, later-type stars having lower temperatures.

System Distance: The estimated distance between the Earth and the planetary system in light-years. The distances between stars are so vast, that they are often measured in light-years, a unit of distance that corresponds to the distance light travels in one year; 1 light-year is 63,240 Astronomical Units, 9.5 x 10¹² km, or nearly 6 trillion miles. For reference, our Milky Way galaxy is about 100,000 light-years across and most exoplanetary systems are in our galactic neighborhood. Sometimes only a rough estimate of the system distance is known.