Planet Birth
Inga's Video Hello (1.7Mb mpg)
Hi, my name is Inga Kamp. I’m a European Space Agency scientist at the STScI. I am discovering how planets form around stars. I do this by using data from HST and other observatories to build a computer program that can be used to understand the environment in which planet formation happens.
This challenge will let you use images form the Hubble Space Telescope to make a presentation about how planetary systems may have formed. You can hunt for other images on the internet to make your presentation even better.
You could make your presentation electronically or on paper.
The Story of the Birth of a Planetary System
Up until 1995 only 9 planets were known - the nine circling the Sun. By mid 2004 there were over 120 planets discovered around other stars with more being discovered at the rate of one a week. Not only have we found planets but with HST we can see how they are forming around young stars.
HST images clouds of dust and gas illuminated by the light of young stars
Stars form out of a cloud of dust and gas called a nebula. Gravity pulls in the matter, probably triggered by the shockwaves from a star exploding at the end of its life or when galaxies pass each other.
Here in the Orion Nebula, material that is
not gravitationally held by a newborn star is cleared away by ultraviolet
radiation given off by nearby massive, bright stars. The stars and their forming
planetary system are blobs of nebula suspended in space. Each one is
up to 1500 astronomical units across. They even have a blobby name - proplyds. Could a proplyd contain the whole of our solar system?
A proplyd in the Orion Nebula revealed by HST
The chances are that the cloud will be spinning one way or another. As it collapses the spin gets faster, just like a dancer increasing a pirouette by pulling in her arms. This flattens the cloud into a saucer shape called a protoplanetary disk.
The magnetic fields of the star blast jets of matter from its poles so that, face on, the central star starts to become visible.
This is the point at which I get my computer programs to model the systems. I can use them to study the chemistry going on in the disk.
Now there
is a race against time. Dust in the disk starts to clump together.
At first it forms small boulder sized lumps called planitesmals.
These then clump together into larger lumps. HST has imaged
these disks and seen clumping and warping - both indicating planet formation.
Disks appear to have a hundredth of the mass of the Sun. Is this
enough to make a solar system like ours?
In early stages, the UV radiation penetrates only the outermost layers of protoplanetary disks. The inside is opaque and does not see any of the stellar radiation. Taking the time scales usually applied to planet formation and various observational indications, the grains start to grow in this very young phase. As the material clumps together it forms small bodies called planetesimals. Some of these fail to form planets and remain in the system as a type of meteorite called carbonaceous chondrites.
The inside of a meteorite gives clues about how planets form
But the time that planetary systems have to form is limited. At some point the star starts to affect the disk. Radiation (infra red, light and ultraviolet and x rays) from the star blows away first the gaseous material and then the dust grains. The radiation can also alter the chemical composition of the disk. If the disk is cleared before planets have time to form, planetary systems like our own solar system might be rare. Investigating the transition between gaseous protoplanetary disks to dusty debris disks is one of the most interesting parts of my research.
Systems like the one around the star Beta Pictoris could already harbour planets or at least huge rocky cores (km-sized bodies).
An understanding of these processes in the disk can help explain the structure of our solar system with rocky inner planets and gassy outer planets. To picture the process, try using a technique called chromatography. Put black ink in the centre of a piece of filter paper (a coffee filter will do). Drip water onto the ink. As the water spreads out it separates the different dyes it contains. The smallest dye particles go furthest and the largest travel the least.
In a similar way radiation from the central star spreads out the material in the protoplanetary disk. The small particles of gas go much further than the much larger particles of dust. You could include a chromatography disk in you presentation as an example of the process.
In the formation of our planetary system, the material inside about 4 AU (Astromomical Units - the average distance between the Earth and Sun) was too warm to form ices, like water, and carbon monoxide. Therefore most of it stayed in the gas phase. These gases were blasted outwards whilst rock particles were left in the inner system. Gravity then pulled this debris together forming first planetesimals and eventually planets before the young Sun cleared the remainder with it's intense radiation.
Mercury was formed from the remaining rock particles (Image by Mariner 10 spacecraft)
The ices beyond 4 AU contained a huge amount of the gaseous material and this material was frozen out and incorporated with rock in the planetesimals. Inside 4AU, the planets could only form from rocky material (which is a low fraction of the total available mass, because most of the mass is in hydrogen, helium, carbon, oxygen and nitrogen). Further out than 4AU, a lot of hydrogen, carbon and oxygen got incorporated into the solids in the form of ices. So there was much more material to build up planetesimals and planetary cores. Only in the outer regions of the solar system could cores grow to huge sizes (about 10 Earth masses). These massive cores were then able to attract the surrounding gas by gravitation and start the process of building gas giants. That's why the first four planets in our system are rocky whilst the next four are gassy.
HST view of our biggest gas giant
Not all planetary system are going to be like the solar system. If you clear away the gas before you grow your planetary cores up to10 Earth masses, there is nothing left to attract in the gases and hence no giant planets could form.
A star's neighbours can also clear its protoplanetary disk. This is happening to some proplyds in the Orion Nebula. The intense radiation from giant stars in the heart of the nebula create bow shocks in nearby proplyds as they are eroded away.
A proplyd bow shock formed by radiation from a nearby giant star
In our system the rocky debris of the Kuiper belt probably failed to make a true planet core and mop up gases. The remaining material in the furthest reaches of the old proplyd resulted in the icy remains of the proposed Oort cloud which is where long period comets are thought to come from.
There is still lots we do not understand about the formation of planetary systems, but instruments like the HST are helping us find out more.