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Exoplanet Detection
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Exoplanet Transits
ADS16402 is a system of 2 solar-type stars that is about 400 lightyears from Earth.  One of the stars (the one that is slightly dimmer) has a companion planet orbiting it.

It was recently discovered that this planet transits along the line of sight from Earth to the star once each orbit (astro-ph/0609369).

A Columbia postdoc, Nestor Mirabal, was at the MDM 1.3m telescope in Arizona on September 19, when a transit occurred.  He took a time-series of flux measurements of the dimmer star.  A Columbia graduate student, Dave Spiegel, analyzed the data as it came in.

This is the H-alpha lightcurve of the star with the companion planet.  As the planet moves in front of the star, the stellar flux is diminished by ~1.5%, which indicates that the planet's area is about 1.5% of the star
's area, or that the planet's radius is somewhat more than 10% of the star's radius.  This means that the planet is a bit larger than Jupiter, and, since its mass is less than half Jupiter's, its density is quite low.
ADS16402 transit

There are several puzzling features in this data.


The most obvious is that the flux level is higher post-transit than pre-transit.  This is probably an instrumental effect that could
be eliminated if more stars were visible in the field.

The two circles labeled A show surprising blips in the lightcurve (plot of flux vs. time) immediately before and immediately after transit.  These blips might well be the result of chance variations in the measured brightness and might have nothing to do with the astrophysical system, but the fact that the blip appears both before and after transit is intriguing.

The circle labeled B points out a blip in the middle of the transit.  This blip might also be statistical, not astrophysical.  But it is interesting to note that this mid-transit blip is consistent with what would be seen if the planet moved in front of a dark star spot (the analog of a sunspot).  This is because if the planet were to move in front of a dark spot, the part of the stellar disc unobscured by the planet would be brighter than when the planet is not in front of a dark spot.  Did we observe evidence of a small dark spot on the star?  Since this was a 1-time observation, and the star spot, if real, will have moved or or disappeared by the next time we take a transit lightcurve, we unfortunately won't be able to confirm or reject this conjecture.








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