Why didn't Johannes Kepler use data about more planets?

Question

In Astronomia Nova (1609) Johannes Kepler used observations of Mars in order to refute circular orbits. Throughout Astronomia Nova he hardly even mentions other celestial objects than Mars, Earth and the Sun (and the Moon in a different context). Were there any astronomical reasons for him not to continue, or even beginning, his investigation by using data about other planets?

Tycho Brahe had collected corresponding data about the other planets too, right? 20 years of data is enough for Saturn to make 2/3 of its period. Jupiter and Saturn have almost twice as frequent oppositions as does Mars. And Mercury has twice the eccentricity and its much shorter period should help getting data about its upsides when it is best seen.

• Would it have helped him to use several planets during the process as he describes it in Astronomia Nova?
• Was it simply the calculation capacity which limited him from dragging several planets into it?
• Or could he somehow conclude that it is enough to solve for Mars, to know that his physical conclusions of interests are valid for all planets?

Answer 1

I know that quoting Wikipedia is frowned upon here, but as there has been no other answer posted, this is what the Wikipedia article on Kepler has to say about the matter:

He then set about calculating the entire orbit of Mars, using the geometrical rate law and assuming an egg-shaped ovoid orbit. After approximately 40 failed attempts, in early 1605 he at last hit upon the idea of an ellipse, which he had previously assumed to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fit the Mars data, he immediately concluded that all planets move in ellipses, with the sun at one focus—Kepler's first law of planetary motion. Because he employed no calculating assistants, however, he did not extend the mathematical analysis beyond Mars. By the end of the year, he completed the manuscript for Astronomia nova, though it would not be published until 1609 due to legal disputes over the use of Tycho's observations, the property of his heirs.[Caspar][Koyré]

[1] Caspar M., Kepler, pp. 131–140;

[3] Koyré A., The Astronomical Revolution, pp. 277–279

Answer 2

Kepler could, with a lot of work, apply his analysis to other planets of course - but it won't help in proving his point of elliptical path. There are only two planets where this elliptical path could be demonstrated effectively given the accuracy of observation of that time. Those are Mars and Mercury; the rest of the planets are simply too far and/or with low-eccentricity which would demand greater observation accuracy. But Mercury is only good in theory, in practice, however, due to its proximity to the Sun (and the low-altitude refraction issues), it made the observation rather difficult. So in practice only Mars was suitable for this mission. Kepler himself knew that very well, when he wrote in Ch. 7 of his Astronomia Nova (translation : William Donahue)

I therefore once again think it to have happened by divine arrangement, that I arrived at the same time in which he [Christian, Tycho's assistance] was intent upon Mars, whose motions provide the only possible access to the hidden secrets of astronomy, without which we would remain forever ignorant of those secrets.