Yesterday I did the following shot of the moon using a 400mm with a 2X:
Now, I would like to know if it's possible to shot pictures of planets using just a tripod?
This is the first time I'm trying to deal with astrophotography.
You can shoot all of the visible planets (Mercury, Venus, Mars, Jupiter, and Saturn) from just a tripod. But since their angular sizes are so much smaller than the moon, don't expect them to fill the frame to the same amount or to get the same amount of surface detail as you can with the moon.
Jupiter and the moon shot from a tripod using an EOS 7D + Kenko C-AF 2X Teleplus Pro 300 + EF 70-200mm f/2.8 L II IS. (35mm/FF Field of View of a 640mm lens)
That is Jupiter to the right of the Moon. At 100% the major atmospheric bands are just visible. Focus was manual using 10X live view with Jupiter as the focus point.
Exposed at ISO 200, 1/125 sec, f/8. Tripod, mirror lockup, and cable release. Digital Photo Professional. Cropped from 5184x3456 to 2172X1448 (now we are at about a 1500mm FoV with a 35mm/FF camera), Monochrome Picture Style, 5200K, -1/3 exposure, Contrast +1, Highlight 0, Shadow -1. Luminance NR 2, Chrominance NR 3. Unsharpen mask: Strength 8, Fineness 8, Threshold 3. Then downsized to 1536X1024 for web viewing.
See also the images in this answer to a different astrophotography question.
It should be possible for you to photograph at least Venus and Mars.
If you use a sky map, it becomes lots easier to prepare the shooting. http://www.space.com/16149-night-sky.html
There you can find the current possitions of the planets and genereal tipps for astro-photography
While technically speaking it is possible to image planets with just a camera, lens and a tripod, it is generally impractical. Planets are significantly smaller than the moon. While the moon looks fine @ 800mm, a planet, even a large one like Jupiter, will generally appear just as small dots with barely-visible features (assuming you can actually get the exposure right).
Planetary imaging generally requires very high magnification. Here, we are talking about large reflecting telescopes like an SCT (Schmidt-Cassegrain Telescope), something in the f/10 range, 8-14" apertures, with native focal lengths in the 2000-4000mm range. This is significantly longer than an 800mm lens.
On top of that, good planetary imaging requires even more magnification than the native f/10 f-ratio of the average SCT will give you, even a large 14" one. So a barlow or extender is usually used, usually a 2x, 2.5x, or 3x. So if we take the 11" SCT (fairly commonly used for planetary imaging), which has a 2800mm native focal length and f/10 native focal ratio, and multiply it by 2.5x, you end up with a 7000mm f/25 scope! An 8" (much more affordable than the 11" or 14" SCTs) would end up being 5080mm f/25. A large high f-ratio scope like this works best on an equatorial tracking mount, however it is possible to use alt/az arm or fork mounts as well (common with more cost-effective consumer-grade telescope packages from Celestron, Meade, etc.) You can also make do with a 6" SCT, however high f-ratio imaging is best done with the largest aperture you can get your hands on.
At this kind of magnification, you can finally magnify the planets to a reasonable size to be imaged at a reasonable level of detail. There is still an additional requirement to get good detail on a planet: high speed video. Planetary imaging at higher f-ratios of f/20 to as much as f/40 is primarily seeing limited. A majority of DSLRs can only read out the full sensor frame at full resolution at around 20fps. That is too slow, and will result in atmospheric turbulence blurring the details. To really image a planet well, you want frame rates no slower than 50fps, and most of the higher detail images you will see are acquired at 100-300fps.
You can find high quality planetary cameras from companies like ZWO or QHY. They cost around $300-500, have tiny yet highly sensitive pixels (often as small as 2.4 microns), and can operate at ludicrously high frame rates when using custom ROI (region of interest)...easily 300fps, and with certain cameras as high as 700fps or so. For a planet like Jupiter, 100fps is enough, for something like Venus 300fps might be necessary to deal with the brightness. For fainter/more distant planets like Mars, Saturn, Uranus you might need 50-100fps.
For some examples of high f-ratio, large aperture planetary imaging, look no farther than Damien Peach...probably the world's best land-based planetary astrophotographer:
His insights go far deeper than my own. ;)
Regarding planetary imaging with a large telephoto camera lens. If you could find a way to stack teleconverters, or just find a high multiplier converter such as a 3x. That might be enough to get you an ok magnification. If, for example, you paired an 800mm f/5.6 with a 3x TC, that would get you a 2400mm f/17 "telescope". The planet would be small, but it might be large enough to get some interesting details. For example, on Jupiter, the red spot should be decently resolved.
The biggest issue with planetary imaging with a lens like that would be the camera. A DSLR is just not going to give you a fast enough frame rate. Especially at f/17....you are going to need longer exposures with a DSLR, as they just don't come close to being sensitive enough. The average modern planetary imager from ZWO, such as the ASI178, has over 70% Q.E. (quantum efficiency, true sensitivity) and a mere 1.4e- read noise at high gain...and can operate at hundreds of frames per second.