The orbits of the planets, including Mars, are eliptical, not circular. Keplers observed positions did not fit a circular orbit. The differences led him to discover that the orbits were not circular, but eliptical.
Kepler defined the basic laws of orbital motion, and these naturally had to be calculated.
Johannes Kepler
I'm pretty sure you're thinking of Johannes Kepler.
Heinrich Kepler was the father of famous German astronomer Johannes Kepler
There is no single planet named Kepler; rather Kepler is a prefix added to the designations of planets discovered by the Kepler telescope. Several planets discovered by Kepler, including Kepler-438b, Kepler-442b, Kepler 440b, and Kepler 296f, orbit in the habitable zones of their stars, which means they might have liquid water on their surfaces. Currently we do not have the technology to determine if they actually have liquid water.
Johannes Kepler
Kepler defined the basic laws of orbital motion, and these naturally had to be calculated.
Johannes Kepler
As of 2014, not enough is known about Kepler 22b. Specifically, its mass is not known; therefore, its surface gravity can't be calculated either.
Kepler showed that three simple statements (Kepler's 'Laws') could explain all the planetary motions that Tycho had observed and recorded. Sir Isaac Newton ... after postulating the law of gravitation ... showed that the existence of gravity, in the form he wrote it, would naturally lead to Kepler's Laws.
According to Johannes Kepler's first law of 1618 the planets each move in an elliptical orbit with the Sun occupying one focus. He discovered that after a lot of work on observed data, largely from Tycho Brahe.
Johannes Kepler
There are lots of planets in the Kepler series. To answer your question, we need to know to which number Kepler you are asking about.
Johannes Kepler determined that all planets have elliptical orbits.
There is no planet that is simply called "Kepler". The Kepler spacecraft discovered various planets, with names such as "Kepler-4b", "Kepler 11-d", etc.
NASA's space telescope "kepler"
Johannes Kepler proposed that these planets orbit the sun in ellipses, not circles. That is why we have Kepler's Law of Planetary Motion.