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
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
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