However, while the field of the formation of planetary systems is a young one, it is worth bearing in mind that, unlike x-ray astronomy in 1970, it is a field with a history. The point here is that just over 10 years ago, the field of planet formation was in really good shape. At that time, theorists were able to predict that if one starts with a disk of gas about 30 AU in radius, with a mass of around 0.01 solar masses, around a star of about one solar mass, then the inevitable outcome is the formation of a Jupiter-mass planet at around 5 AU, a Saturn-mass planet at around 10 AU, an Earth sized, terrestrial-type planet at around 1 AU, and so on. The success of this picture has had a very strong influence on our thinking, and it is good to retain an awareness of this fact, in case our preconceptions start to lead us astray. To give an example, some 15 years ago, Doug Lin and I wrote a paper on the "Initial evolution of protostellar disks." Our disks started life as massive (a good fraction of the mass of the central star) and large (a few hundred to a thousand AU). We got ridiculed by some of the pundits at that time, because, we were told, it was clear from the Solar System that disks are neither that massive, nor that big. Thus, when I hear talk of "hot Neptunes," without any evidence that the objects in question are anything like Neptune, I start to worry a little. And to hear one speaker talking glibly about 'other solar systems' sets alarm bells ringing. The concern here is the extent to which perversion of the language can lead on to muddled thinking.
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