Listen to part of a lecture in an astronomy class. Before 1995. Professor: Before 1995 the only planets we knew about were those in orbit around our own sun. So everything we thought we knew about planets was based on them, the planets here in our own solar system. The big question was, how did the planets form and the prevailing theory back then was the core accretion theory. According to the core accretion theory, When the sun formed in the middle of a huge flat spinning disk of gas and dust, there were some leftovers. Stuff that didn't go into making the sun. And as the disk kept swirling around, bits of matter collide and some stuck together in clumps. And over millions of years, some clumps gathered a created more and more material and eventually grew large enough to form rocky cores. But due to the sun's energy over long periods of time, the cores of planets closer to it could it hold on to light gases very well, so they remained small rocky planets like earth. Meanwhile, though the planets farther out from the sun, their gravitational force kept on pulling in gas molecules, these colder planets could create and hold onto huge amounts of gas and you ended up with giant planets such as Jupiter composed largely of gas. The final result of this accretion is what you see in our solar system. Rocky planets nearer the sun and gas giants farther out and then there's the Pluto which will just ignore for the time being. In 1995 a new planet was dis in orbit not around our sun, but around a different sun, a star called 51 PE. This was not just a great first in itself, but the Discovery of 51 pegasy B that's what they named this first extrasolar planet. This immediately challenged the core accretion theory of planet formation. That's because 51 peggy B is the gas giant that orbits very close to its sun. In fact, 8 times closer to its sun, the mercury, which is the closest planet to our sun, 8 times closer and a gas giant, which theory would not have predicted so near. Clearly, a different theory of planet formation was needed. Hence the disk instability theory. The disk instability theory had been around for a while, but hadn't gained much attention until the discovery of 51 pegacy B. The theory basically says that if the gaseous disk around a newborn star is massive enough and cold enough, then any unevenness of gravity within the disk could cause clumps even giant clumps of gas is to form in as little as 1,000 years. And those plants could then become gas giant planets. So the disk instability theory says giant planets could form near as without needing a long time to do it. fFnny, isn't it? How one little one momentous discovery can change what we thought we knew was true. Well, as if it wasn't enough to have these two competing theories of planet form. Another one somewhat related to the core accretion theory was then proposed. Some astronomers remain unconvinced that a gas giant planet could form so close to a ssun among other things, they doubt that gases would last very long there because the sun's intense radiation could just blow them away. So they support the idea of orbital migration. The orbital migration theory suggests that a gas giant planet forms far from its sun but then over time moves to a position even closer to that sun than where you'd expect to see smaller rocky planet. And the powerful gravity of such a large planet holds onto its massive at even when it's so close to that sun. So we find this one gas giant planet orbiting very close to its sun. And then it turns out that 51 pegacy B is not at all unique. That is just the first of a number of large gaseous planets we found that we like to call hot Jupiters because they're about the size of Jupiter or larger and close enough to their sons to be very, very hot. So if all we believed about planet formation was based on our own solar system, it turns out that our solar system lacks at least one fairly common type of planet. Then what does that say about what we really know?