Exoplanets are planets that orbit a star different from our own sun.
In some cases the orbit of that planet is such that it covers it’s star – as seen from Earth – once every orbit. Because a planet doesn’t emmit light itself, it will as a result block a tiny percentage of the star’s light.
This ‘dip’ in star brightness can be measured. The frequency gives the orbit timing, and the depth of the brightness dip gives the planet’s size.
tres 1b is a planet very close to the size of Jupiter. It orbits a orange dwarf star of magnitude 11.8 in the constellation of Lyra @ a distance of 523 light years.
I got lots of help from Pieter Vuylsteke. The observation was logged in Exoclock
More information on this planet on Wikipedia