On this weblog submit, we clarify the which means of, and the right way to use, the completely different values and data given within the topic metadata on Planet Hunters NGTS.
On Planet Hunters NGTS, when you classify a topic you may select ‘Performed’ to maneuver to the following topic and preserve classifying, or you may select ‘Performed & Speak’ if you wish to begin a dialogue with the Planet Hunters group about something fascinating you’ll have discovered. When you’re taking a look at a topic picture within the Speak dialogue boards, you’ll have the ability to entry the topic metadata by clicking the ‘i in a circle’ button. This metadata is more information concerning the topic, a few of which will probably be fascinating to Zooniverse customers eager to delve deeper into their classifications.
When you click on the metadata icon, you’ll discover a various variety of info fields, relying on whether or not the topic has linked photographs or is a recognized planet, however extra on that later.
The primary discipline is ‘sde’ which stands for ‘Sign Detection Effectivity.’ This worth might be interpreted as how “robust” the sign is, in keeping with the pc algorithm. The calculation of this worth is described in Kovács et al. (2002) which describes the Field-fitting Least Squares (BLS) algorithm that varieties the premise for the NGTS exoplanet transit searches.
As an illustration, take into account a full mild curve (not phase-folded) such because the picture beneath (which makes use of synthetic knowledge). I’ve injected a transit with a 3-day interval into the sunshine curve and we’re going to make use of a BLS algorithm to seek for this.
I’ve used the AstroPy bundle’s BLS periodogram device. This calculates the ‘energy’ for a variety of durations, this ‘energy’ is equal to our SDE worth talked about above. The ensuing ‘periodogram’ is plotted within the determine beneath. This exhibits the facility worth plotted in opposition to interval with peaks within the periodogram comparable to the place the algorithm believes there’s a robust periodic sign within the mild curve.
As we are able to see, the algorithm has picked out the strongest sign at a interval of three days, simply as we anticipated as that is the true interval of the sign I injected. We additionally see peaks (of reducing energy) round 1.5 days, 6 days, 9 days and 1 day. Chances are you’ll discover that these are all multiples or fractions of the true interval. We refer to those as interval aliases and it’s anticipated that they’ll produce peaks within the BLS periodogram. Generally these may even be the true interval of a sign, which leads us to our subsequent metadata discipline.
‘peak’ is the energy of the height in keeping with the BLS algorithm, ranked by SDE. 1 is the strongest peak (our 3-day interval within the instance above), down to five for the fifth strongest. Something much less important than this doesn’t have a phase-folded picture generated for Planet Hunters NGTS. Presently solely Peak 1 objects are on the Planet Hunters NGTS website as these are almost certainly to correspond to the true interval of a transiting object however we plan to incorporate plots for different peaks in future.
‘prod_id’ and ‘obj_id’ are inner identifiers which can be used as labels for the celebrities. These values subsequently solely have which means to the NGTS science group.
‘plot_type’ merely refers to which workflow this topic is from:
- ‘primary_phased’ is for topics within the Exoplanet Transit Search. These plots present the place we expect the first eclipse of the phase-folded mild curve is. See Determine 1.
- ‘secondary_phased’ is for topics within the Secondary Eclipse Verify. These plots present the place we expect the secondary eclipse may very well be, round part=0.5. See Determine 5 beneath
- ‘odd_even’ is for topics within the Odd Even Transit Verify. These plots present the odd (1st, third, fifth, and so forth.) transits in inexperienced and the even (2nd, 4th, sixth, and so forth.) transits in magenta. If the depths of the odd and even transits don’t match then that’s a transparent indicator that we’re taking a look at an eclipsing binary fairly than an exoplanet transit. See Determine 6 beneath for the Odd/Even picture comparable to the topic proven in Determine 1.
‘stellar_rad’ is the radius of the host star, expressed in models of Photo voltaic radii (i.e. a star with stellar_rad equal to 2.0 has a radius twice the dimensions of our Solar). This can be utilized to estimate the radius of the transiting object. First, we estimate the depth of the transit. For the topic proven in Determine 1 (which is confirmed planet NGTS-5b), the transit depth is round 0.025 (or 2.5%), because the flux drops from 1.0 to 0.975 (typo corrected). We are able to then use this, together with the stellar radius of 0.75 Photo voltaic radii, within the equation:
to calculate an estimated planetary radius of 1.15 Jupiter radii. You possibly can learn extra about how planet radius, stellar radius and depth are associated right here. The equation I exploit here’s a faster methodology because the multiplication issue of 9.73 is such that should you use R* in photo voltaic radii, as given within the metadata, then your reply will probably be in Jupiter radii. You need to use this methodology to work out whether or not a transiting object is plausibly a planet, sometimes something above 1.5 Jupiter radii is unlikely to be a planet.
(Notice typically the stellar radius worth will say ‘nan’ or ‘unavailable,’ this simply means we don’t have measurement of the star’s radius.)
Additional fields
‘primary_phased,’ ‘secondary_phased’ and ‘odd_even’ present hyperlinks to the completely different plot varieties for topics which have handed into the secondary and odd/even workflows. This makes it simpler to vet candidates as you may view all of the obtainable knowledge shortly and simply.
Lastly, we have now the ‘known_planet’ discipline. It will solely seem for beforehand recognized exoplanets (reminiscent of NGTS-5b above) and signifies that the topic picture you might be viewing corresponds to a planet that seems within the NASA Exoplanet Archive. Discovering recognized exoplanets is a helpful check of the detection efficiencies of the challenge as an entire, and it’s at all times thrilling to know should you’re taking a look at knowledge from an actual exoplanet system too! Some topics on the Planet Hunters NGTS website are nonetheless present process follow-up by the NGTS science group so could be robust planet candidates however gained’t be confirmed and printed exoplanets. Which means there will probably be topics not marked as recognized planets of their metadata, however the NGTS group might already pay attention to it and have put within the work to start characterising the system additional.
We hope this weblog submit can function a helpful reference in future as you get entangled with Planet Hunters NGTS.
