Name the first planet to be reached by a space probe?
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Venus, and it was the Soviets (Russians) who got there first, with their Venera missions in the 1960s and 1970s.
231566
mariner 2 14/12/62 successful fl-by\nvenera 4 18/10/67 sampled atmosphere\nvenera 7 15/12/70 sent data from surface\nmariner 10 5/2/74 fly-by on way to mercury\nvenera 9 23/10/75 first orbit and soft landing and first surface image \nvenera 15 10/10/83 first radar mapping\npioneer-venus 2 9/12/78 multiple descent probes investigate atmosphere\nmagellan 10/8/90 complete radar mapping
661023
The Planetary Grand Tour was an ambitious plan to send unmanned probes to the outermost planets of the solar system. Conceived by Gary Flandro of the Jet Propulsion Laboratory, the Grand Tour would have exploited the alignment of Jupiter, Saturn, Uranus, Neptune, and Pluto, an event that would occur in the late 1970s, and not recur for 176 years. A probe sent to Jupiter could use that planet as a gravitational slingshot to extend its trajectory to Saturn. After repeating the maneuver at Saturn, Uranus, and Neptune, the probe would complete its 12-year voyage with a flyby of Pluto.
164779
we already found a 10th planet in our solar system, but it's still unclear if it's considered as planet (some say it is too small).\nThe name of the "10th planet" is Sedna.
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New planet, discovered by the Samuel Oschin Telescope at the Palomar Observatory near San Diego, Calif., on Jan. 8, 2005,  is yet to be formally named.\n\n(This  newly-discovered object is more distant than the mysterious planetoid SEDNA discovered in 2003. Sedna is designated 2003 VB12.)\n\n Is at least as big as Pluto and about three times farther away from the Sun than Pluto. It is very cold and dark.It is currently about 97 times farther from the sun than Earth, or 97 Astronomical Units (AU). For comparison, Pluto is 40 AU from the sun. \n\nThis places the new planet more or less in the Kuiper Belt, a dark realm beyond Neptune where thousands of small icy bodies orbit the sun. The planet appears to be typical of Kuiper Belt objects--only much bigger. Its sheer size in relation to the nine known planets means that it can only be classified as a planet itself
364339
P \nfor Pluto \nThe letters are the planets starting with the first one from the sun
217292
Mercury is the closest planet to the Sun. the asteroid belt is located between the 5th planet (Mars) and the planet Jupiter
469045
The first was Apollo 11...
289819
Not a satellite, since that's something that is in an established orbit, but a space probe, yes.  There's one on the way at the moment.  At full speed, it'll be the fastest traveling manmade object ever, but even at that sort of speed it'll take nine years to get there.
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Neil Armstrong was the first.\nBuzz Aldrin was the second.
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The most recent planet discovered in our our solar system is named Cedna after the Inuit goddess of the sea. It is even smaller than Pluto.
984027
An ion engine will get you there the fastest. You get the biggest delta-vee per kg of exhaust. \n\nProblem is, in general you only get one encounter with a specific planet once each solar orbit, and you can only get a kick so large each time. I suppose you can hypothesize a super convenient planetary alignment that could shave off some time, but you might spend a long time waiting for that particular configuration.  A lot of really smart people spent a lot of time developing the computational tools to come up with the VEEGA and VVEJGA options.  I doubt the population here is going to come up with anything a lot better.\n\nYou can read more about the DS1 ion propulsion experimental system here:\nhttp://nmp.nasa.gov/ds1/tech/ionpropfaq.html\n\nThe first science mission using ion propulsion is the Pluto bound New Horizons.  Thanks to this tech, it will reach pluto in 10 short years and reaches Jupiter for a gravity assist in only 1 year.\n\nhttp://pluto.jhuapl.edu/mission/mission_timeline.html
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astronomers are trying to define the word planet, but it isn't easy.  many astronomers favor the reclassification of the bodies in the solar system into three groups, the rocky terrestrials, the gas giants, and the ice dwarfs.  there are hundreds of known ice dwarfs, including pluto, quaoar, sedna, 2003 EL61, and 2003 UB313 (the official name of xena).  this is also called the kuiper belt.  pluto was the first kuiper belt object discovered.  2003 UB313 is about five percent bigger than pluto.\n\nhttp://en.wikipedia.org/wiki/Kuiper_belt
61531
The next mission to Mars is the Mars Reconnaissance Orbiter. The MRO will make high-resolution measurements of the surface from orbit. It will also search for underground water, which may be a home to primitive life.
9741
By size and orbit, mainly.\n\nOrbit comes first. Kepler noticed that the gap between Mars and Jupiter was wider than could have been expected, Johann Titius described a systematic and J.E. Bode formulated a rule for the distances of the planetary orbits from the sun. This rule works fine for the inner planets, the missing planet at the asteroid belt, Jupiter, Saturn and Uranus, but places Neptun on a intermediary orbit (about half the distance expectedby the Titius-Bode rule).\n\nWhen Pluto was discovered, its mean distance was about the expected value for the planet behind Uranus, which is why it was classified as a planet.\n\nWith the discovery of Sedna, Pluto now is regarded as a Kuyper Belt object rather than a regular planet.\n\n\nAsteroids come as numerous bodies occupying broadly the same orbit around the sun, without a central planet.\n\n\nBTW, the Earth-Moon system is an oddball in itself, basically a binary planet with the gravity centre between earth and moon forming the planetary orbit around the sun.\n\n\nSize, or surface gravity, is the other criterium. There is no fixed value where a planet begins and planetoids end, but 1/10th of earth gravity could be a useful rule of thumb.\n\nSeveral of the larger moons (earth's, Titan and at least for of Jupiter's moons) would qualify as planets if they had their own solar orbits rather than orbiting a heavier planet. Most of the smaller moons are de facto asteroid/planetoids caught by the  planets.
741583
You are right.  Neptune and Uranus have only been visited by Voyager 2.  Jupiter and Saturn have been visited by multiple probes however.
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That would be OGLE-2005-BLG-390Lb, a planet we belive to be orbiting around the star OGLE-2005-BLG-390L.  There is debate on if this detection is really a planet, as the technique used to find it was a one-shot event (gravitational micro-lensing), and can not be repeated.\n\nIf it is a planet it would be the most distant one we have ever found with a distance of between 18,200 and 24,800 light years away from us, in the direction of the galactic center.