UR: Searching for Outer Companions to Hot Jupiters

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Tim Crowe

University of Warwick

This guest post was written by Tim Crowe. Tim is an undergraduate studying Physics with Astrophysics at the University of Warwick. He completed this research as a part of the URSS scheme at the University of Warwick under the supervision of Dr. David Brown. He plans to present these results at the URSS poster showcase.

Stars observed by TESS with previously discovered planets were selected and narrowed down by limiting the search to systems with already discovered Hot Jupiters (period <10 days, mass >0.4 Jupiter masses; using data from the NASA Exoplanet Archive) and with at least 10 TESS sectors. TESS (Transiting Exoplanet Survey Satellite) is an orbital telescope designed specifically to locate exoplanets outside our Solar System using the transit method, whereby a planet eclipses its parent star, showing a measurable dip in the observed flux. Each TESS sector is a 27-day period of observation covering a rectangular area of sky (24° by 96°), there are 13 of these carried out each year and the hemisphere covered alternates between north and south each year.

The TESS data were modelled using the Lightkurve Python package, a periodogram was constructed for each star and the most dominant period was analysed to locate the Hot Jupiter. The period and transit time were extracted and a Box Least Square (BLS) model was constructed. These transits were then masked out of the light curve simply by subtracting the BLS model from the original light curve. This process could then be repeated to find a smaller, longer period outer companion to the Hot Jupiter but further analysis must be carried out using Monte Carlo simulations (random sampling algorithms) in the Exoplanet Python Package, these simulations randomly change certain parameters to find the best model using estimated values of each parameter. In this project, these estimated parameters came from the Lightkurve periodogram analysis. A folded light curve, whereby the extracted transits were plotted on top of each other could be constructed, and then a corner plot was made to test the precision and accuracy of each Monte Carlo parameter.

The best candidate was a transit around WASP-119. If the modelled transit is later confirmed to be a planet through further observations of extrapolated transit times, the candidate exoplanet (WASP-119c) would have an orbital period of 198.8 days, through periodogram analysis; a radius of 5.4 Earth radii, found by obtaining the ratio from the folded light curve in the figure (A) and comparing with WASP-119’s already calculated radius; and a semi-major axis of 0.71 AU, found using Kepler’s third law (assuming the mass of the exoplanet is negligible compared to the star, which is likely). This means that if this WASP-119c candidate is confirmed as a planet, then it would be considered a so-called super Earth due to its size.

(A) Folded light curve of the WASP-119c candidate. (B) Corner plot of the WASP-119c candidate, where ror is the radius ratio between planet and star, and b is the impact parameter.

The corner plot in the figure (B) shows that the period and radius ratio (ror) of the planet and the star are in good agreement, shown by the random trials being concentrated in the centre of the square aligning well with the blue guide. But the impact parameter (b), which is a measure of how far from the centre of the star the transit is, could be anything between 0 and 1. This means that it could transit anywhere from across the equator of WASP-119 or just skimming in front of the poles of the star. This should not happen and implies there is an error or aberration causing this, however the transit is too promising to discount.

Because of this source of doubt, further observations are required to confirm the presence of a new exoplanet at an extrapolated point in time when another transit will occur. This would be best done through ground observations rather than attempting to find a future TESS sector that observes the correct patch of sky at the correct time.

Astrobite edited by: Ali Crisp

Featured image credit: NASA

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