Protoplanetary disks


The discovery of protoplanetary disks around young stars

After Isaac Newton explained the motions of planets in our Solar System using his Laws of Motion and Gravity, scholars tried to explain how our planetary system might have originated.  The model that proved successful was envisioned by Immanuel Kant in a book Universal Natural History (1755); he later became famous as a philosopher (rather than a scientist).  Kant proposed that our Solar System condensed out of a gaseous disk orbiting the Sun, long ago.  This makes sense:  All of the planets in our Solar System orbit the Sun in the same direction and in (nearly) the same plane ... so they were probably born in a flattened disk resembling a dinner plate with the Sun in the middle.  

Immanuel Kant's model for the Solar System was largely forgotten for many years, and astrophysicists thought about star formation mainly with a simplistic spherical model.  Here gas and dust in interstellar clouds undergo gravitational collapse, falling directly into the core where a star forms.  We thus expected protostars to be enshrouded in a spherical envelope of dust and gas.  This envelope has, in fact, recently been seen using millimeter telescopes.

But the flattened structure orbiting the protostar, the protoplanetary disk where planets can form, eluded detection until the 1980-90s.  Three steps can be traced in their discovery and study:

1. IRAS (InfraRed Astronomical Satellite) detected strong infrared emission around young stars. IRAS was a collaboration between NASA, the United Kingdom and the Netherlands in the 1980s.   Infrared astronomy had been in a very primitive state due to the difficulty of designing sensitive detectors and the infrared brightness of the Earth's atmosphere, even during nighttime.   When the first infrared telescope was launched into space, it found that some stars had tens, hundreds or thousands of times more infrared emission than expected.  The excess was found in intermediate-mass stars like Vega, and in young stars in molecular clouds.  The best explanation was a flattened disk of dusty gas at temperatures around T ~ 100-500 K (degrees Kelvin, the Earth's surface is at T=300K).  However, the IRAS telescope detected the infrared emission from the disks, it did not focus well enough to image the disk, and did not have high spectral resolution to study the composition of the disks.

2. In the early 1990s, NASA's Hubble Space Telescope observed some these nearby young stars and clearly imaged the disks.  Three types are shown below:  the AB Auriga and HD 141569 disks are seen in reflected starlight; several disks around protostars in the nearby Taurus molecular cloud are seen in silhouette against the illuminated gas above and below the disk; and disks around young stars in the Orion Nebula Cluster are seen in silhouette against the bright emission behind the stars.  These were the first time astronomers directly saw the protoplanetary disks where planets form.  


Hubble Space Telescope image of the protoplanetary disk around the young solar-like star AB Aur.  The black wedge covers the bright emission from the central star to reveal the fainter light reflected off the disk. Hubble Space Telescope image of the disk around the young star HD 141569.  There is a hit of a gap in the disk:  such gaps are predicted by astrophysical models when a Jovian proplanet has swept out material in a ring within the disk.  There is one case where a protoplanet has been imaged in a disk, but it is not seen here.  




3. In the mid-2000s,  NASA's Spitzer Space Telescope observed hundreds of protoplanetary disks.  Spitzer still could not focus well-enough to image the disks, but had an excellent infrared spectrometer to obtain detailed spectra of the protoplanetary disks.  Three examples are shown below.  The AA Tau disk shows water (H2), carbon dioxide (CO2), hydrogen cyanide (HCN), and acetylene (H2C2).  The HH 46 disk shows ices (CO2, H2O, and alcohol CH3OH), gas (CH4 methane), and rock (silicates).  The CRBR 2422.8-3423 disk shows gasses (CO, CO2, and NH3) , rock (silicates), and ices (water). Spitzer thus showed the composition of disks are closely related to the composition of planets. 

Read more about these Spitzer Space Telescope spectra of protoplanetary disks:  AA TauHH 46; and CRBR 2422.8-3423.  




We conclude that -- using space-borne telescopes like IRAS, Hubble and Spitzer -- 

astronomers have discovered where planets are born: 

in flattened protoplanetary disks that orbit every known protostar 

in star forming regions nearby in the Milky Way.  

These protoplanetary disks have all the chemical ingredients to make planets

like those orbiting our Sun, and the millions of planetary

systems orbiting other stars.