Criticality of the Radius Measurement of Exoplanets

by Dr. Tahir Yaqoob on April 19, 2012

Two of the most basic and important things you would want to know about an exoplanet are its mass and size. Whilst most of the (currently) 763 exoplanets have a mass estimate, less than a third have a radius measurement. Getting a handle on the size of an exoplanet is harder than getting the mass estimate because it requires observations of transits (or eclipses) of the host star. Knowing or not knowing the radius has all sorts of repercussions. Knowing the size, you can get an estimate of the density, and thereby crude constraints on the planet’s compostition and therefore its “type.”

A preprint that appeared a few days ago illustrates how critical the radius measurement of exoplanets is. It also illustrates how sometimes you have a result one day, but gone the next. The paper, entitled Non-Dectection of Transits of the super-Earth HD 97658b with MOST Photometry, by Dragomir et al. is submitted to a peer-reviewed journal but not yet accepted. (Note: MOST is a Canadian space telescope, and photometry is simply the measurement of light levels.) Essentially, the paper is about an attempt to verify and confirm previous claims by another group of eclipses by the exoplanet HD 97658b classified as super-Earth. The classification as a super-Earth was hinged entirely on its radius measurement (about 3 times that of Earth). Given its mass estimate of at least 8 Earth masses, the exoplanet was thought to be a low-density super-Earth, but distinct from the gas giant class of planets because of its size.

Well, when the observations were repeated by the other group (Dragomir et al.) the eclipses were nowhere to be found. The paper explains how the previous observations of the transits were likely to be artifacts, possibly due to changes in the diminishing of light by dust along the path. Since the changes in light levels due to genuine transits are so tiny, it is a very dangerous business.

So with the transits gone, the radius measurement vanishes just like that, and so does the planet’s classification as a super-Earth: it could be much larger than 3 Earth radii. Not only that, the mass estimate (from the wobbling of the host star) is only a lower bound now because the constraint that the orbital plane is viewed edge-on, or nearly edge-on, also vanishes because that orientation is a prerequisite to observe eclipses in the fisrt place. Mass estimates of exoplanets including the orientation of the orbital plane relative to the observer as an unknown (and observations of transits eliminate that unknown).

In the end, it’s worth remembering that it’s actually absolutely remarkable that we can even attempt to measure the size of exoplanets at all. Taking the example of a super-Earth 3 times the size of Earth situated 10 light years away (about the distance of the closest known exoplanet), that’s equivalent to trying to measure the height of a person standing more than 5 million miles away from you. Mind-boggling, isn’t it?

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