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Planetary orbits are elliptical, and the deviation of the ellipse from a circle is characterized by the *eccentricity*. The length of half of the longest axis of the ellipse is called the semimajor axis, and can be thought of as 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 closest exoplanet to its parent star that has been found so far, 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 shortest semimajor axis (i.e., the exoplanet that is closest to its parent or host star), you want the semimajor axis measurements to be ascending, with the smallest values appearing first. (Note that at the end of the sorted table the rows 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 closest to 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 closest to its parent star was formally PSR 1719-14 b. This planet has a radius of 0.4 Jupiter radii, a minimum mass estimate of about 1 Jupiter mass, and a nearly circular orbit with a semimajor axis of only 0.0044 AU (that's only about 658000 kilometers, or 409000 miles). This is an incredibly short distance for something as massive as Jupiter. However, notice that the host star is not a regular star, but a pulsar (hence the “PSR” in the name). A pulsar is a star in which nuclear fusion has ceased and the object has collapsed under its own gravity to become a highly compressed mass of neutrons. A pulsar is a spinning neutron star.

If we want to find the exoplanet that is closest to “regular” star, we look at the next object on the list. On 4 August 2012, the next two objects were listed as having the same semimajor axis values, and these exoplanets were KOI-55 b and Kepler 42 c. The semimajor axis is given as 0.006 astronomical units (AU), which is about 0.9 million kilometers, or about 0.56 million miles. This is still a remarkably short distance, and there is no counterpart to such such close-in exoplanets in our own solar system. The orbital periods for KOI-55 b and Kepler 42 c are about 0.24 and 0.45 Earth days respectively.

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.

See also the exoplanets semimajor axis distribution.

Read more about exoplanets in Exoplanets and Alien Solar Systems.

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

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