Wednesday, June 1, 2011


A couple of decades ago the only planets known were those in our own solar system, but at an ever increasing rate new planets are being discovered around other stars - exoplanets (or extrasolar planets).  Fortunately there is a superb, and free, app that keeps you up to date with this rapidly changing field, Exoplanet.  It is one of the best designed science apps that I have seen, and will appeal to those with just a curious interest in this field or those who do research in the field.

It is first and foremost a frequently updated (incorporates push notifications) database with detailed information on each exoplanet (it is based on the publicly available Open Exoplanet Catalogue, maintained by the app author, Hanno Rein). For example, it includes such information as the orbital parameters and estimated mass and radius of the exoplanet, as well as the mass, distance, spectral type and metallicity (to astronomers metallicity refers to the fraction of the mass in elements other than hydrogen and helium) of the parent star.  One of the best features is that it has links to the actual scientific publications, and/or preprints on the arXiv preprint server, that describe the discovery.

But it is so much more than just a database.  The visualizations are superb.  If you click on a specific exoplanet that was discovered using transit techniques it shows you the light curve dip as the planet crosses the parent star, a rotating visualization of the exoplanet compared to solar system planets, a star map showing the parent star and a planetary system plot for the parent star with the habitable zone, the region where water based life could survive, shown in green.  If you click on an exoplanet discovered using the RV (radial velocity) method the simulation shows the plot of the orbital motion of the exoplanet and star, and a graph of the Doppler spectroscopy for the discovery.

I also like the correlation diagram section that allows students to look at how two parameters compare.  For example is eccentricity correlated with orbital period for exoplanet discoveries.  This is a powerful tool for students to study systematic biases in exoplanet data, as well as to look for trends that may give hints about planetary system formation.  The parameters that can be correlated include the orbital parameters semi-major axis, orbital period, eccentricity; the physical parameters mass, radius; stellar parameters such as distance and metallicity; and other factors such as discovery year.

There is also a Milky Way feature that allows you to zoom in and to see the precise location of exoplanets within a galactic context.

I have used the Exoplanet app on hardware ranging from a first generation iPod Touch (needs 3.13 operating system) to an iPad 2, and it works flawlessly on both.  As expected, the user experience is much better on the iPad where it takes full advantage of the larger screen.

So who is the author of this amazing app?  Dr. Hanno Rein is a researcher at the Institute for Advanced Study in Princeton, after having completed a PhD at Cambridge with the Applied Mathematics and Theoretical Physics group (his thesis title is "The effects of stochastic forces on the evolution of planetary systems and Saturn's rings" and was defended late in 2010).  You can see a list of his research publications using the NASA ADS system.

Rating: 10 (out of 10) - yes it really is that good!

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