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# What Exoplanet is the Farthest from its Parent Star So Far?

The deviation from a circular orbit of a planet can be characterized by the eccentricity of an elliptical orbit. The length of half of the longest axis of the ellipse is called the semimajor axis, and corresponds to the maximum star-planet distance. Since the semimajor axis is the parameter that is most often given in data tables (as opposed to the semiminor axis, which is the shortest star-planet distance), we will examine the semimajor axis as a measure of the star-planet distance when comparing the collection of confirmed exoplanets.

If you would like to be able to find out yourself what is the farthest exoplanet to its parent star that has been found so far, and what its properties are, here is how you do it. Go to the Extrasolar Planets Encylopedia catalog page and click on the column that is simply called a (in August 2012 it was the fourth column after the exoplanet name, but this may have changed). When you click on the column header, the data in the table should be sorted in order of numerical value in that column (i.e., the semimajor axis of the planets). An arrow just to the right of the column header tells you whether the numerical order is ascending or descending (the arrow will point upwards for an ascending order). If you click the column header again, the numerical ordering will flip from ascending to descending or vice versa.

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To find the exoplanet with the largest semimajor axis (i.e., the exoplanet that is farthest from its parent or host star), you want the semimajor axis measurements to be descending, with the largest values appearing first. However, at the beginning of the sorted table there will be rows that will have no entries in the semimajor axis column, and these correspond to exoplanets for which the semimajor axis has not been measured. The first non-blank entry in the sorted semimajor axis column corresponds to the confirmed exoplanet that is the farthest from its parent star. The number in the cell in the semimajor axis column is a distance in astronomical units (AU). From here you can convert to any other units by typing an appropriate statement in a Google search box.

As an example, on 4 August 2012, the exoplanet farthest from its parent star was WD 0806-661B b. This planet has no radius measurement, a minimum mass estimate of about $8\pm2$ Jupiter masses. It has a semimajor axis of 2500 AU (that's about 374 billion kilometers, or 232 billion miles). This is an incredibly long distance and is nearly 4% of a light year. The Extrasolar Planets Encyclopedia gives no margin of error on this value of the semimajor axis. No orbital time period was given for WD 0806-661B b. The exoplanet was detected by direct imaging.

Remember that, because of Kepler's thrid laws, larger star-planet distances imply longer planetary orbital times. Orbital periods for a semimajor axis of 2500 AU are expected to be of the order of 125000 years for a sun-like star, and about 161000 years for the mass of the star in the WD 0806-661B system. However, there is no way these long periods can be measured directly by humans at this time, so the orbital period has to be inferred.

A word of caution: you should always try to find out what measurement uncertainties are associated with any quoted number (not just for exoplanet semimajor axis values, but any measurement in general). However, the measurement uncertainties on various parameters are not often given, even in the Extrasolar Planets Encyclopedia: you have to dig a little deeper into the literature.

File under: What is the largest distance between an exoplanet and its parent or host star so far? What exoplanet has the largest semimajor axis? How can I find the latest information on the exoplanet with the largest semimajor axis?