When trying to find exoplanets, the form of the transit can inform us rather a lot about what object we might be . For the Planet Hunters NGTS exoplanet transit search, we ask you to establish if a transit is U-shaped or V-shaped, in addition to whether or not there’s stellar variability, information 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 under). 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) reveals a transparent V-shape produced by an eclipsing binary system. On this case, the transiting star solely partially eclipses the goal star, which means that it passes throughout the sting of the disk of the goal star however by no means passes totally 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 lowering 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 stage (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 can be virtually vertical. The rationale V-shaped dips have angled sides is as a result of the item blocking out the sunshine is usually (however not at all times!) one other star. The eclipsing star is massive (in comparison with a planet) so takes extra time to go totally in entrance of the goal star, subsequently the lower in flux occurs over a big time interval and we get an angled ingress (likewise for the egress because the star stops blocking mild).
The angled sides are extra pronounced in Determine 1, however don’t be fooled by dips with a curved base like Determine 3 under! If the transit has angled sides then it’s nonetheless a V-shape! The curved base of the transit is brought on by a phenomenon referred to 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 spread of phases.
How vertical is vertical? Sadly there isn’t a transparent reply to this, which is why we use human vetting somewhat than simply a pc to examine these mild curves. The instance under (Determine 4) is the sunshine curve for confirmed exoplanet HATS-43b, which was categorised as U-shaped by all 20 volunteers who considered it. This can be 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 implies that it virtually 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 may produce a really shallow V-shaped dip, subsequently we’ll get spherical to looking these classifications for potential exoplanets too! The edges of the dip seem extra angled as a result of shorter whole period of a grazing transit; it’s very possible that the size of the x-axis on the plots (the part) will present a a lot smaller vary of numbers resulting from how brief these transits can be. The ingress and egress occasions can be just like a daily transit however the central dip is way 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 mild curves under for a near-grazing transit by WASP-174b (Determine 5). The dip has angled sides as a result of zoomed in x-axis and has a curved base resulting from limb darkening. This can be a traditional curvy V-shape, but it surely’s additionally an actual exoplanet!
There isn’t a definitive reply for when a curvy V turns into a daily U-shape, however as at all times your instinct and greatest guess is what we wish! We hope this weblog publish makes it simpler to identify the variations between U- and V-shaped dips once you’re classifying mild curves on the Planet Hunters NGTS web site, and keep in mind you possibly can at all times examine the Area Information or ‘Want some assist with this process?’ for extra assist. There’s additionally the group of researchers and moderators on the ‘Discuss’ boards who can be pleased to assist!
Sean & the Planet Hunters NGTS Group
