Star and Planet Formation

Dr. Dana E. Backman

My talk today is about star and planet formation. The real issue we are getting into is whether there are planetary systems around other stars. People can use the phrase solar system and planetary system interchangeably, but really solar system refers to our set of nine planets plus assorted other stuff around our Sun because the proper name of our Sun is Sol - S O L - so when you say solar system you mean ours.

Planetary system is the more general term meaning the idea that other stars might have planets around them. The importance of this of course is whether the Earth is unique or whether planets like the Earth with life on them are common in the universe. And surprisingly we don't know the answer to that though people make a lot of assumptions. And there is kind of a dogma you might be familiar with that these systems are everywhere. And the reasons for this are that we have reason to expect that when a star forms, there will be normally left over material around it arranged in the form of a planetary system.

But, we have to check. You can't finish a process with what you think is reasonable. You have to go looking for these things. And so one of the things I hope to lead up to is the idea of a good place to go looking for other planetary systems in the process of forming.

Now, when we look out into space we see material between the stars which is called the interstellar medium - sometimes abbreviated the I.S.M. That stuff is gas and dust between the stars, and you can see it because it blocks the light from stars to some extent. It makes distant stars look dimmer and also redder than they normally should. The reddening and the blockage of light is due to the dust, little bits of solid material in the interstellar medium.

There is also gas, and what we have learned about what this stuff is made out of is that it is made out of the same stuff that the Sun and the planets in our solar system are made out of. The Sun is mostly hydrogen and helium plus little bits of other stuff - familiar substances, oxygen, iron, calcium. You name it, it is in the Sun although the Sun is mostly hydrogen and helium.

The outer planets - Jupiter, Saturn and so on - those are made out of substances very similar to the Sun, whereas the Earth and its siblings - Venus, Mercury and Mars - are made out of just the dusty stuff, leaving out the hydrogen and helium.

This stuff that is arrayed out in outer space - clouds of gas and dust - would tend to naturally contract because of its own gravity. And there might be opposing forces. For instance, if the material is too warm or if it has a magnetic field threaded through it, that might oppose the contraction. But by some process we don't completely understand, it is obvious that there are places in space where this material gets swept up. And the clouds of gas and dust begin to contract.

Eventually you will get a star when the material has contracted so much that it's heated up, and the interior is so hot that nuclear reactions can start. So we see plenty of star-forming regions all over space, localized regions where gas and dust have been concentrated and stars are condensing out of that material.

Some kinds of stars which only last a short period of time are so luminous - that's why they last only a short period of time - they are so luminous that they light up these regions, and they are easy to see across great distances of space. Now with infrared and radio telescopes we can actually look through the dust which is usually obscuring, or had previously been obscuring, and actually see embedded deep inside of these star clouds stars still in the process of forming: imbedded protostars they are called.

Well then, how do the planets form? As I said, there are sort of two types of planets - the Jovian planets that are like Jupiter, and the terrestrial planets that are like the Earth. That's important to figuring out how the solar system formed. Also, the solar system is flat and most of the motion in the solar system is all in one direction around the Sun.

These facts together lead us to believe that the solar system, - the planets - formed out of a flat spinning disk of leftovers from the process of the Sun forming. It makes sense theoretically that this should happen. If you do the calculation of what should happen as a big cloud starts to contract, you should get this "pancake" of leftover stuff. A physics calculation can tell you that.

But we also observe in space things called protostellar disks around stars like T Tauri, a variable star in the constellation Taurus, which are opaque pancakes of material, gas and dust concentrated around a star that is not quite finished forming. These stars are usually - as far as we can tell - younger than maybe ten or twenty million years. That sounds like old, but that is much younger than the Sun which is about five billion years old.

Then, as far as we understand, the process should go on for that opaque disk and the material in it to combine and stick together to make planets. Then the disk itself would disappear and you would be left with a transparent solar system as we have now.

Do all stars form planets? We wish we knew. We have to go check and these are exciting times because we are right now, in the 90s, in the middle of finding this out. But it's tougher than you think to check. The glare from a star is so strong that we can't see the small planets right next to the star, even with the space telescope. Believe it or not, the space telescope can't do this.

And the tiny effects that the planets have on the star, like make the star wobble back and forth as the planets go around, we might be able to detect that. But it is a very tough problem. So far, two planets have been found elsewhere in space, but they are going around a pulsar which is a burned-out star. So that changes the situation. Those planets probably aren't anything like the planets in our solar system. They formed by some other means.

At least we know that some planets can form in some situations. So we are in the process of trying to find planets around other stars though it is a very, very tough technological problem. A way to sneak up on the problem is something that was offered to us by the IRAS satellite which was to go look for remnant material like gravel and dust and sand orbiting nearby stars which would be left over from the planets forming.

It may seem surprising, but it is actually easier to detect lots and lots of small material, like sand and smaller size grains, than it is to detect the planets. When the planets formed, the surface area of a planet is so tiny, it is very tough for us to detect from a long way away. But the raw material or the leftover material from a planet forming is much easier (to detect).

In 1983 the IRAS satellite started finding that normal stars nearby in the same stage of evolution as the Sun had this small particle material arrayed around it out beyond where planets might be. Some of the stars were Vega, which is Alpha Lyrae, and Beta Pictoris. Beta Pictoris, which is in the southern sky, was found to have a very easy to photograph disk of dust around it.

It is not opaque, not like the ones that are earlier stages. We think this is a later stage where the planets have mostly formed and the dust has accumulated. The photograph showed this disk of stuff around a star which is finished forming. Beta Pictoris is completely formed and is in its normal main sequence life.

The nearest region where stars of all sizes are forming is in Orion's sword - the Orion star-forming region which is a naked-eye star below Orion's belt in Orion's sword around the nebula M42. There is a huge molecular cloud which is opaque and can be detected with radio telescopes. There is what is called an H-II region which is lit up like the gas in a neon tube. It is ionized gas that is glowing with these pretty pink and lavender colors.

And bunches of young stars, so young that in this case if there are disks around these stars that are forming that are going to be planetary systems, they are probably still in the opaque stage, well before the Beta Pictoris and Vega stage, more like the star T Tauri.

This would be a good place to go check on statistics of how often stars make planets because you can see how often stars have these leftover disks around them.


Originally published in NASA COTF ASTRONOMY VILLAGE CD-ROM 1995 as Star and Planet Formation, by Dr. Dana E. Backman