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The Traveling Exhibit. Alien Earths. Science Background Part B: Star & Planet Formation. prepared by Dr. Cherilynn Morrow for the Space Science Institute Boulder, CO. B. Star and Planet Formation. KEY QUESTIONS: Where do stars & planets we observe in the galaxy come from?

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TheTraveling Exhibit

Alien Earths

Science Background

Part B: Star & Planet Formation

prepared by Dr. Cherilynn Morrow for the Space Science Institute Boulder, CO

b star and planet formation
B. Star and Planet Formation


Where do stars & planets we observe in the galaxy come from?

Around which kinds of stars should we look for signs of life?

we are star stuff the astronomical spectroscopy of sir william huggins

Huggins’ private observatory Tulse Hill, London

We are “Star Stuff”The Astronomical Spectroscopy of Sir William Huggins

“One important object of this original spectroscopic investigation* of the light of the stars and other celestial bodies, namely to discover whether the same chemical elements as those of our earth are present throughout the universe, was most satisfactorily settled in the affirmative; a common chemistry, it was shown, exists throughout the universe.”

- Sir William Huggins

*In 1859, Huggins began observing the stars using Bunsen's and Kirchhoff's discoveries that spectral emission and absorption lines could reveal the composition of the source.

star and planet formation
Star and Planet Formation
  • Planets sometimes form along with a central star out of the same swirling disk of gas and dust.
  • Our search for life beyond our solar system requires knowing where and how this process occurs.
  • The best chance to find an “Alien Earth” might be to look around stars like our Sun.
  • The “birth” of star systems where life evolves depends on the “death” of massive stars that spew out elements like carbon & oxygen.

Where do stars and planets come from?

Artist Concept:

Early star-planet system in formation

“proto-planetary disk”

searching around sun like stars
Searching Around Sun-Like Stars
  • We know that in our case, a planet (Earth) with life has evolved around such a star.
  • High-mass stars (> 10 times the mass of the Sun) burn brightly with ultraviolet radiation that tends to destroy proto-planetary disks. Massive stars also have relatively short “lifetimes” (1 to 10 million years as compared to 10 billion years for the Sun). This is not time enough for planets with life to evolve.
  • Low-mass stars (< 0.6 times the mass of the Sun) are relatively dim and have very long lifetimes: there is time enough for life to evolve around them. Their dimmer light sometimes makes observations more challenging.

Why search for planets and life around Sun-like stars ?


When Sun-Like Stars “Die”

Butterfly Nebula

Eskimo Nebula

Cat Eye Nebula

Stars have “life cycles”. They are “born” and they “die” but are not alive like us.

Stars like the Sun “die” by “puffing” off their outer layers of gas and dust. This process creates a beautiful variety of NEBULAE in the Milky Way GALAXY.


When Massive Stars “Die”

  • Blue giant stars that are much more massive than the Sun form elements heavier than hydrogen & helium (e.g. carbon, nitrogen, oxygen) via nuclear fusion in their cores.
  • Such massive stars“die” in incredibly powerful explosions called supernovas.
  • Supernovas spew heavier elements needed for life out into the galaxy.
  • The explosions themselves can stimulate new star birth by sending shock waves into nearby clouds of gas & dust.

Stars Have “Life” Cycles

Crab Nebula: Supernova Remnant

the right stuff
The Right Stuff

Supernovas help ensure that heavier elements and complex molecules are found throughout interstellar space.

  • Since the “building blocks” of life are so utterly common, perhaps it might not be so strange to find life everywhere!
  • And yet, the living cell is so amazingly complex that we must wonder if it commonly emerges and survives, even on other Earth-like worlds.

Forming Worlds Where Life Can Exist

Artist’s concept of the formation of OUR SOLAR SYSTEM

1. Something (perhaps a supernova) triggers the gravitational collapse of a nearby interstellar cloud. The cloud naturally heats up and spins faster as it collapses. Collisions between particles flatten the cloud into a disk.

2. The Sun and planets start to form in this spinning, flattened disk (proto-planetary disk), with the Sun at the hottest central part.

3. In our Solar System, Earth formed in the inner region of the disk where rocky & metallic material could condense in the greater heat. Ices & hydrocarbons settled in the outer regions where gas giants like Jupiter form.

4. Computer models tell us that Jupiter’s gravity played a strong role in causing comet & asteroid impacts to supply water & organic materials to Earth from the outer solar system, thus contributing to its habitability.


Star Forming Regions

These large clouds of gas and dust in our Milky Way galaxy are the types of regions where many stars are forming: Orion and Eagle NEBULAS.



Our entire solar system would fit in this small nub

hst images of proto planetary disks
HST Images of Proto-planetary Disks

Proto-planetary disks around young stars in the Orion Nebula.

The disks range in size from 2 to 8 times the diameter of our Solar System.

spitzer sees through the dust
Spitzer “Sees” through the Dust

Where do stars and planets come from?

  • Spitzer is the largest infrared telescope ever launched into space.
  • Many areas of space are filled with vast, dense clouds of gas and dust which block our view.
  • Infrared (IR) light, can penetrate these clouds, allowing us to peer into regions of star formation and into newly forming planetary systems.

Artist Concept:

NASA’s Spitzer Spacecraft

now you see stars now you don t
Now You See Stars, Now You Don't
  • The image composite compares an infrared image taken by NASA's Spitzer Space Telescope to a visible-light picture of the same region (inset).
  • The added detail in the Spitzer image reveals a dynamic region in the process of evolving and creating new stellar “life”.
  • Spitzer is both seeing, and seeing through, the dust.

Overhead Infrared Camera

looking down at nebula

The monitor displays what the IR camera is “seeing”

Our hands look red on the screen. We emit IR light!

Yes! I wonder what’ll happen if we put our eyeglasses under the IR camera?

Heat source beneath the nebula on the round table

Alien Earths:

Infrared (IR) Camera Interactive

Cone Nebula

star forming region

There are round heat sources beneath the nebula on the table which cannot be seen with the human eye, but which are being “seen” by the infrared camera and recorded on the monitor. The IR light “shines” right through the table! In a similar way, the Spitzer Telescope can detect infrared radiation shining through dusty interstellar clouds revealing young stars in formation.

design a solar system
Design a Solar System

Alien Earths:

Design a Solar System

Touch Screen


“Hey, I can drag a bunch of planets in orbit around a star and then let them zoom around to see if they will fly out of the system, or crash into each other, or be gobbled up by the star!”

  • Computer models show that not every configuration of planets is a stable one.
  • The gravitational effect of a Jupiter-sized planet in a close elliptical orbit can be to kick terrestrial planets out of the system.
  • Jupiter-sized worlds in more distant circular orbits are much more helpful to the formation and evolution of Earth-like planets in a habitable zone.

Based on real, multi-body model simulations of the stability of planetary orbits.


B. Star and Planet Formation

Elements heavier than Hydrogen & Helium form inside stars and then are released back into the galaxy when stars “die”. Thus materials that are the “building blocks” for life & Earth-like planets are found throughout interstellar space.Planets sometimes form along with a central star out of a common swirling disk of gas and dust. Our search for life beyond our Solar System requires knowing where and how this process occurs. Perhaps the best chance to find an “Alien Earth” is to look around stars that are most like our Sun.