Exoplanet Eccentricity vs Orbital Period

[Drakkith]

I have a question concerning the eccentricity vs orbital period of observed exoplanets. Going to this link let's you plot different exoplanet properties on each axis of a graph. Plugging in Orbital Period for the X-axis and Eccentricity for the Y-axis shows that a trend towards decreasing eccentricity with shorter orbital periods. I'd just like to know why exoplanets (and other objects I suppose) have less chance of having a highly eccentric orbit when their orbital periods are very short.

Thanks.

By the way, the link above (http://exoplanets.org/plots) takes you to a super-cool database where you can plot any of a few dozen exoplanet properties and compare them. I highly recommend it to anyone interested in super-cool things. Especially that Phinds guy. Go! Plot things! Be super-cool like this guy is--->

 

[Bandersnatch]

My guess would be tidal effects. Tidal interactions tend to circularise the orbit, unless some resonance interferes. Shorter orbital periods mean tighter orbits and/or more massive central body, which translates to increased tidal effects on the planet.

 

[mfb]

For very close orbits, tidal effects would be my guess as well.
It could also be some selection bias. Planets with short orbital periods were mainly observed via transits, planets with longer orbital periods via radial velocity. I can imagine that the latter method profits from a larger eccentricity a bit as it leads to larger and more sudden velocity changes. You are also sensitive to different planet types - radial velocity mainly finds gas giants.

 

[Drakkith]

Planets with short orbital periods were mainly observed via transits, planets with longer orbital periods via radial velocity.

Shouldn't the radial velocity method be more effective for finding exoplanets with short orbital periods rather than long orbital periods?

 

[mfb]

Sure, but there the transit method lead to hundreds of smaller planets, so those hot jupiters are a smaller group now.

Did you check your plot with additional constraints on the mass? Larger/smaller than some value?

 

[Drakkith]

I've filtered the results to show just the planets detected by the radial velocity method and it still appears to show a trend. See the attached picture.