Measurement of the transit light curves of the exoplanets WASP-84b and KPS-1b on March 7 and 8, 2024

April 24, 2024, Dr. Gerold Holtkamp

The measurement of the transit light curves of the exoplanets WASP-84b and KPS-1b is described. The radius of the exoplanets is determined from the degree of “darkening” of the parent stars. The transit method makes this possible, although the two systems can only be observed as point sources of light.


At the beginning of 2024 there were hardly any clear nights in Osnabrück. But the night of the 7th/8th March and also the night of 8th/9th March (at least until about 1 a.m.) were cloudless. The first night was calm. In the second, however, there was a moderate wind, which is why, as a precaution, the dew shield of the Newton telescope was removed to provide less wind resistance. The moon played no role on both nights, as there was already new moon on March 10th. The temperature was -1°C.

Measurement setup

The equipment setup was the same on both nights except for the changes mentioned above:

Telescope: Skywatcher Newton 250/1200 mm
Mount: Skywatcher AZ-EQ6
Filter: Luminanz (Antlia)
Camera: QHY268M with gain 60, offset 20, chip temperature -10° C
Guiding: Off Axis Guider with PHD2

AstroArt 8 was used for camera control.

Transit light curve of WASP-84b on March 7, 2024

The star WASP-84 is 328 light-years away from Earth. It has 0.77 times the radius of the Sun and a surface temperature of 5300 K. It appears to us to be about 10.8 mag (Transit Finder) / 10.377 mag (ExoClock) bright and is located at the head of the constellation Water Serpent. [1] [2] [3]

WASP-84b was discovered in 2014. It orbits its star at a distance of 0.0775 AU (Earth-Sun distance). It takes 8.5 days to complete one revolution. It has 0.95 times the radius of Jupiter with a mass 0.69 times Jupiter. Its surface temperature is given as 732 K. [3]

In 2023, another planet was discovered in the system, WASP-84c. It only has about 2 times the radius of Earth, which means that its transit light curve has too little of a dip to be detected by amateur telescopes. [3]  

The Measurement

Between 19:10 and 23:01 UTC, 296 images were taken with 40 s exposure time each. To normalize the recordings, 10 flats, darks and flat darks were created. The normalization and evaluation of the data was carried out using HOPS, the software provided by the ExoClock project. [4] The measurement data was published in the databases of ExoClock, ETD and AAVSO. [5] [6] [7]

Transit light curve of KPS-1b on March 8, 2024

The star KPS-1 is 857 light-years away from us. Its apparent brightness is given as 12.97 mag in the NASA Exoplanet Archive. Its surface temperature is approximately 5,000 K. It got 0.9 times the diameter and about 0.89 times the mass of our Sun. KPS-1 is located in the constellation Ursa Major, near the star Dubhe. [8]

KPS-1b was discovered in 2018. It orbits its parent star in 1.71 days at a distance of 0.0269 AU. No information could be found about the shape of its orbit. It got the same size as Jupiter (1.03 times). Its mass also corresponds to that of our Jupiter (1.09 times). On its surface it is around 1,450 K hot, no wonder given the proximity to its parent star. It is the first exoplanet discovered by amateurs. [9]

The Measurement

Between 20:12 and 0:30 UTC, 97 images were taken with 150 s exposure time each. This almost four times longer exposure time compared to the measurement with WASP-84b reflects the significantly lower brightness of KPS-1. To normalize the recordings, 10 flats and flat darks and 5 darks were created. The normalization and evaluation of the data was also carried out using the above-mentioned HOPS software. The measurement data are published in ExoClock, ETD and AAVSO databases.


In ExoClock's evaluation report, the result is given directly as the ratio of planetary radius to star radius, i.e. Rp/Rs. With a known star radius, which is determined using other methods, the planetary radius* is obtained. [10]

For the exoplanet Wasp-84b you get, with a star radius of 522,000 km, a measured planet radius of 60,700 +/-3,760 km. [3] Admittedly, this is not yet a direct hit to the expected value of 67,630 +/-313 km given by ExoClock.

For the exoplanet KPS-1b from the ratio of planetary to star radius obtained through the measurement with a radius of the parent star KPS-1 of 646,900 km, one gets the planetary radius of 74,200 +/-2,782 km while the expectation value by ExoClock is 73,747 +/-2,588 km. [8] A nice match.

This is how one can, as an amateur, determine planetary radii even though you just got point light sources available! This was unthinkable 35 years ago!

For comparison with other measurements, go to the ETD database, where in addition to the transit depth, which is given in mag**, the transit duration and the transit time are also given: For Wasp-84b [11], For KPS-1b [12].

Finally, a little imagination will certainly make the measurements described a little more clear - with the help of NASA.



[1] https://astro.swarthmore.edu/transits/transits.cgi/

[2] https://www.exoclock.space/

[3] https://exoplanetarchive.ipac.caltech.edu/overview/wasp-84b/

[4] https://www.exoworldsspies.com/en/software/

[5] https://www.exoclock.space/database/observations/WASP-84b_1036_2024-03-07_Gerold_1561_Lum/


[6] http://var2.astro.cz/EN/tresca/transit-detail.php?id=1711027634


[7] https://app.aavso.org/exosite/?obscode=HGEA&in_notes=&star_name=&exoplanet_name=&start_date=&end_date=&results_per_page=&search=true/

[8] https://exoplanetarchive.ipac.caltech.edu/overview/KPS-1b

[9] Publications of the Astronomical Society of the Pacific, 130:074401 (10pp), 2018 July https://iopscience.iop.org/article/10.1088/1538-3873/aabde2/pdf

[10] https://www.weltderphysik.de/gebiet/universum/astronomische-massstaebe/sterngroessen/

[11] http://var2.astro.cz/ETD/etd.php?STARNAME=WASP-84&PLANET=b

[12] http://var2.astro.cz/ETD/etd.php?STARNAME=KPS-1&PLANET=b

[13] https://exoplanets.nasa.gov/exoplanet-catalog/6293/kps-1-b/

[14] https://de.wikipedia.org/wiki/Weber-Fechner-Gesetz