When trying to find exoplanets, the form of the transit can inform us quite a bit about what object we might be taking a look at. For the Planet Hunters NGTS exoplanet transit search, we ask you to determine if a transit is U-shaped or V-shaped, in addition to whether or not there’s stellar variability, knowledge gaps or no important dip within the flux in any respect. An exoplanet transiting a star will sometimes produce a U-shaped dip, however there are conditions the place that isn’t the case (extra on that beneath). In the meantime an eclipsing binary (two stars orbiting one another) will produce a V-shaped dip more often than not.
The primary plot (Determine 1) exhibits a transparent V-shape produced by an eclipsing binary system. On this case, the transiting star solely partially eclipses the goal star, that means that it passes throughout the sting of the disk of the goal star however by no means passes absolutely in entrance. Which means the purpose of minimal flux doesn’t final lengthy earlier than the flux begins to extend once more.
The defining distinction between U- and V-shaped dips is the angle of the perimeters of the transit, or ingress and egress to offer them their scientific names. Ingress is when the transit begins and the flux is reducing to the minimal (place 1 to place 2 in Determine 2), whereas egress is when the transit is ending and flux begins to extend again to the traditional degree (place 3 to place 4 in Determine 2).
V-shaped dips have sides which might be at an angle whereas U-shaped transits could have a steeper lower and improve within the flux, a lot in order that the perimeters of the transit will likely be nearly vertical. The rationale V-shaped dips have angled sides is as a result of the article blocking out the sunshine is usually (however not at all times!) one other star. The eclipsing star is giant (in comparison with a planet) so takes extra time to cross absolutely in entrance of the goal star, subsequently the lower in flux occurs over a major time interval and we get an angled ingress (likewise for the egress because the star stops blocking gentle).
The angled sides are extra pronounced in Determine 1, however don’t be fooled by dips with a curved base like Determine 3 beneath! If the transit has angled sides then it’s nonetheless a V-shape! The curved base of the transit is attributable to a phenomenon known as ‘limb darkening,’ the place the central disk of a star seems brighter than the sting. The eclipsing star on this system is not only grazing the limb of the goal star both, which is why the minimal flux of the transit is sustained for a variety of phases.
How vertical is vertical? Sadly there isn’t a transparent reply to this, which is why we use human vetting moderately than simply a pc to verify these gentle curves. The instance beneath (Determine 4) is the sunshine curve for confirmed exoplanet HATS-43b, which was categorized as U-shaped by all 20 volunteers who considered it. It is a clear instance of the close to vertical drop in flux for the perimeters of the transit. The small radius of the planet in comparison with its host star signifies that it nearly immediately passes by the ingress and egress phases, in comparison with the time taken by a bigger star in an eclipsing binary system.
However wait! V-shaped dips can nonetheless be exoplanets too! Similar to the partial eclipse that produced the sharp, V-shaped dip in Determine 1, an exoplanet can carry out a grazing transit the place it simply crosses the limb of the star and doesn’t go over the centre of its host star’s disk. This can produce a really shallow V-shaped dip, subsequently we are going to get spherical to looking these classifications for potential exoplanets too! The edges of the dip seem extra angled because of the shorter complete period of a grazing transit; it’s very seemingly that the dimensions of the x-axis on the plots (the part) will present a a lot smaller vary of numbers because of how quick these transits will likely be. The ingress and egress occasions will likely be just like an everyday transit however the central dip is far shorter. The limb darkening impact additionally has a extra apparent impact on the form of the dip, which we are able to see within the gentle curves beneath for a near-grazing transit by WASP-174b (Determine 5). The dip has angled sides because of the zoomed in x-axis and has a curved base because of limb darkening. It is a basic curvy V-shape, but it surely’s additionally an actual exoplanet!
There isn’t a definitive reply for when a curvy V turns into an everyday U-shape, however as at all times your instinct and greatest guess is what we would like! We hope this weblog submit makes it simpler to identify the variations between U- and V-shaped dips if you’re classifying gentle curves on the Planet Hunters NGTS website, and bear in mind you possibly can at all times verify the Area Information or ‘Want some assist with this activity?’ for extra assist. There’s additionally the crew of researchers and moderators on the ‘Discuss’ boards who will likely be glad to assist!
Sean & the Planet Hunters NGTS Workforce