Home → Exoplanets FAQ → What is the longest orbital period of an exoplanet?
Finding the exoplanet with the largest orbital period involves some rather tricky issues. One of the problems is that the largest orbital cannot be measured directly because they are of the order of hundreds to thousands of years. (We have only been studying for less than three decades!) So you can be sure that any claimed orbital period that is longer than duration of time over which an exoplanet has been obsereved is an indirect estimate. A related issue is that the uncertainty in the large orbital periods observed indirectly is typically very large so the lower bound (within the margin of error) of the largest orbital period in a data table may be lower than the orbital period for an object further down the table. On top of all that consider how such large orbital periods are estimated in the first place. Typically, it can usually only be done if the exoplanet is one of the few that can be directly imaged. In that case we can estimate the size of the orbit and then, using Kepler's laws, we can infer the corresponding orbital period. At the time of writing (4 August 2012), it so happens that one of the exoplanets with the largest orbital period has had its direct imaging data questioned to the extent that some researchers conclude that what was thought to be the image of an exoplanet is simply the image of a “random” dust cloud, and not a planet at all (see below).
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If you would like to be able to find out yourself what is the longest orbital period of an exoplanet found so far, here is how you do it. Go to the Extrasolar Planets Encylopedia catalog page and click on the column called Period. When you click on it, the data in the table should be sorted in order of numerical value in that column (i.e., the orbital period of the planets). An arrow just to the right of the word Period 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.
To find the exoplanet with the longest orbital period, you want the period measurements to be descending, with the largest values appearing first. However, not all exoplanets have orbital period measurement so you will see blank entries at the beginning of the period column. Scroll down to the first non-blank entry in the sorted orbital period column. The number in the cell in the Period column is the orbital period in units of Earth days (or 86400 seconds). The first entry is not necessarily the exoplanet with the largest orbital period, however. You need to find out more about the exoplanet in the first entry by clicking on its name (which will be in the first column of the row for that planet). Examine and write down the orbital period and its given margin of error. Repeat for the next few entries in the sorted orbital period column until you establish what the longest orbital periods are, given their overlapping margins of error.
As an example, on 4 August 2012, the exoplanet with the longest period was Oph 11 b. This planet has no radius measurement, a minimum mass estimate of about $21\pm3$ Jupiter masses, and an orbital period of about $730,000 \pm 365,000$ days. This is an incredibly long period: actually about 2000 years give or take 1000 years. So the margin of error is also huge. Now, the next entry in the sorted orbital period column was the exoplanet Fomalhaut b, with a given orbtial period of $320,000$ days, or about 876 years (no margin of error is given). As mentioned above, these huge orbital periods cannot be measured directly, they must be inferred and that requires an image, and currently only a few percent of exoplanets can be image. Fomalhaut b is actually famous for being one of the few imaged exoplanets, and in fact has been a poster-child for an imaged exoplanet. However, here's the problem: the identification of the planet in the image has been challenged and shown to be a mistake from analysis reported in a paper by Janson et al. (2012) (although this result itself has also been disputed but a formal rebuttal had not been published as of 4 August 2012). If the image of the exoplanet is not real, nor is the given orbital period. Moving on to the third candidate in the sorted orbital period column then yields the exoplanet HR 8799 b. The orbital period given for this planet is 164,250 days, or about 450 years (no margin of error is given). Even if the margin of error were as large as 100%, that would give a period that could be as high as 900 years, which is less than the lower bound on the period for Oph 11 b (1000 years). We conclude that the longest known orbital period for an exoplanet is currently somewhere in the region of about 400 to 3000 years. The lower end of this range would be claimed by Oph 11 b and HR 8799 b, whilst the upper end would be claimed by the first of these two exoplanets.
A word of caution: you should always try to find out what measurement uncertainties are associated with any quoted number (not just for exoplanet periods, 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 period distribution.
Read more about exoplanets in Exoplanets and Alien Solar Systems.
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