The first thing I should mention is that the planets themselves, extra solar planets, planets around other stars, were not known until about 15 years ago. But before that we actually had a pretty good idea how planets formed. I would say it was actually about the time I was a graduate student that real progress began to be made on this topic. In fact, when I first came to Berkeley I decided to work on a new topic and there was a professor here who studied very young stars and of course, very young stars should be surrounded by very young planets if they do in fact make planets. So that seemed like a really interesting topic to me. At that time we didn’t know much about these young stars except they appeared to be very young. It was thus over the period between about 1980 and the present that we’ve really learned almost all that we know about planet formation.

1995 was a very watershed year for both brown dwarves and planets. But then people began to argue about when is it a planet and when is it a brown dwarf. It was by coincidence around the same time that arguments about the other end of planets, that is to say, how small can you make a planet before it’s not a planet anymore was also really coming to a boil in the astronomical community. In addition to these discoveries of extrasolar planets and brown dwarves, people had begun to discover objects in the outer part of our solar system that were near the size and mass of Pluto. The question then became first of all, when do you cut this off? When do you stop calling it a planet and then the other problem was Pluto now appeared to be orbiting in a big belt of smaller objects and this had come up before. The last time this had happened, the known object which was the largest asteroid Ceres had been demoted from being a planet. When that was first discovered, it was thought to be the next planet. It was the next planet between Mars and Jupiter. But once it was discovered to be in a belt of lots of objects orbiting with it, people said “aw, that’s not really a planet” and so they demoted it and we now call it an asteroid. So astronomers who understood all of this, some of them began to agitate for demotion of Pluto. To name it for the belt that it was in. The so-called Kuiper Belt. So I’m sure even the public became aware of this controversy through Pluto. But as I said, there were arguments on both ends of this so the question is, is Pluto a planet. How small can something be and still be a planet? Then there were arguments at the upper end which was how massive can I make a planet before it becomes a brown dwarf. This has been a very interesting debate in the astronomical community. It was voted on but I would say not quite resolved by the International Astronomical Union last year. Some astronomers haven’t accepted that decision and in any case the definition that the IAU offered wasn’t intended to be a general definition of planet. It was intended to be a definition of how we should call things in our solar system. So some astronomers and I’m included among them still feel that the general question of what is a planet and how we should define that word is still open to discussion.

The International Astronomical Union took this issue on in 2006 and made a vote to demote Pluto from planet to dwarf planet. But it did not actually try to define what is a planet. So the vote that was taken only really applies to our solar system and the question of how the word planet should really be defined is really still open.

So the exciting discovery was that the young stars like the stars I was studying showed up as being very little dimmed by dust and yet they had a big infrared excess around them so there was a lot of dust there and really the only way that could work is if that dust was concentrated in a plane. So you’re looking at the star from above the disk, so you can see the star directly and you don’t suffer any dimming and yet all that dust is around it and shining out all that infrared light. And by the mid-80s it was clear that indeed around many young stars there were disks or very flattened distributions of this dust and so that told us that this basic idea that stars are surrounded by disks was not only something that happened, but it actually happened fairly commonly. In other words, a lot of the stars that we looked at where we thought this might be going on did in fact show disk signatures. They appeared to be surrounded by a lot of dust and they were not dimmed by that dust.

Galileo would probably find this discussion and these discoveries really interesting. He was really along with Copernicus the person who demoted the earth from being the center of the universe to being merely a planet. I mean this shows that the definition of planet has undergone big changes in the past. The earth was not a planet until Copernicus and Galileo came along. So I’m guessing that they would be happy to find out that many other stars had bodies orbiting around them that were like the planets in our own solar system and that planetary systems were common in the sky. That wouldn’t present a problem for Galileo or Copernicus.

Galileo
Copernicus
Geocentric
Heliocentric

Proprietary disk
Light absorption
Visible light
Infrared light
Dimming of starlight

The trick astronomers were able to use to tell that the dust was actually in a disk form rather than a cloud form is that if you surround a star with a cloud of dust that dust will actually absorb some of the visible light from the star and make it look dimmer and redder than it would otherwise look. So you can measure the visible light from the star and then you could measure the infrared light from the star and then you could predict - If I’m putting all that dust that I’m measuring in the infrared, all around the star, how much dimming should I get?