FEATURE ARTICLE
Spitzer’s Cold Look at Space
To get a clear view of infrared emissions from celestial objects, the Spitzer Space Telescope has been cryogenically cooled—and what sights it has seen
Michael Werner
Protoplanetary Disks
Spitzer’s initial studies of protoplanetary or potentially planet-forming disks concentrated on broadband measurements aimed at tracing, at least statistically, the evolution of the disks over time. These results showed, for example, that the extremely dense and dusty phase of the disks’ lifetime around solar-type stars lasts only a few million years, consistent with estimates of the time required to form gas-giant planets such as Jupiter and Saturn. At the end of this time, many of the small dust particles that were present in the disk as it formed would have coalesced into larger particles, dramatically reducing the dust’s overall surface area and hence its infrared radiation. The formation of rocky planets such as Mars and Earth is thought to proceed over a few tens of millions of years by further coalescence and coagulation of the dust particles.
More recently, research emphasis has shifted towards spectroscopic studies of both the dust and the gaseous component of the disks, as a means of assaying the composition and physical condition of material in exoplanetary systems. Observations of disks in the active planet-forming stages have revealed a rich complex of gas-phase organic molecules, including water, carbon dioxide and carbon monoxide—precursors of the complex organic chemistry that took place on Earth, which eventually led to the formation of life. These observations are unique to the infrared because the principal bands in which such molecules vibrate and produce their spectra fall in this region, and they all are easily excited to do so at the temperature of the protoplanetary material.
Now that spectra of this type are available for literally dozens of disks, it is possible to look for significant variations of the abundance pattern and compare it with other properties of the disk and the central star. One such trend with interesting implications has been the demonstration that the nitrogen-bearing compound hydrogen cyanide (HCN) is less abundant than expected in the gaseous material around forming stars of low mass (having less than half the mass of the Sun) than in more massive solar-type stars. It is thought that the absence of HCN reflects the photochemistry in the planet-forming disk rather than an anomaly in the material from which the star is forming. A possible implication of this measurement is that planetary systems forming around stars in this mass range might be relatively nitrogen-poor. As nitrogen is a major player in biological molecules on Earth, this could suggest that a different set of processes would govern development of life on planets orbiting lower-mass stars than were in play in the terrestrial environment.
Spectroscopy of the dust, in addition to the gas, associated with protoplanetary disks and exoplanetary systems also has been productive. In general these spectra show smooth, continuous emission, punctuated by broad emission and absorption features due to ices or silicate minerals. The implied composition of the dust particles agrees with that of dust spread diffusely through the interstellar medium, but the physical state of the material can vary dramatically. In particular, the silicate material in the interstellar medium appears, based on its spectral characteristics, to be amorphous in form, whereas that associated with planet-forming and debris disks often shows the sharper emission features of crystalline silicates.
On this topic, Spitzer also took advantage of a unique opportunity to study cometary material in our Solar System by obtaining spectra of the aftermath of the 2005 Deep Impact planned collision with the comet Tempel 1, which liberated a cloud of dust and gas from the subsurface layers of the nucleus that persisted for some 40 hours. A main constituent of the dust, as has been seen in other comets in our Solar System, were again small particles of crystalline silicates, very similar to those from exoplanetary material.
Such a result reveals a major puzzle: How do the amorphous materials, which dominate silicates in the interstellar matter from which the star and planetary system formed, convert to the crystalline form that is dominant in planetary systems? It is understood that this requires the amorphous material to be annealed, that is, to be subjected to temperatures greater than 1,000 kelvins, at which the individual molecules in the mineral are free to reorient and reorganize from the amorphous into the crystalline state.
A recently reported Spitzer result gives us our first real insight into the annealing process while also exemplifying the serendipity of scientific undertakings. In this case, a known, young, variable star that had previously been observed by Spitzer brightened by a factor of about 100; this giant flare lasted for several months, after which the star relaxed towards its pre-outburst brightness. An alert group of astronomers noted this phenomenon and were able to observe the star again several months after the initial brightening, but while it was still much brighter than usual.
Remarkably, there was a pronounced change in the character of the silicate emissions between the two observations. In particular, before the outburst the silicate material showed emission characteristic of amorphous material, whereas afterward the emission was crystalline in nature. It appears that the outburst heated the dust at the surface of the protoplanetary disk to above 1,000 kelvins, converting the amorphous material to crystalline form and changing the appearance of the silicate emission. Because such flaring activity is known to occur frequently during the evolution of stars with protostellar disks, it is possible that the large-scale amorphous-to-crystalline conversion results from an accumulation of such flaring events over the 1 million years or so during which the disk is dense and the first stages of planetary-system formation are occurring.
Thus Spitzer contributes to very exciting new research comparing the properties inferred for exoplanetary systems with those of our own Solar System. This activity brings the wealth of detailed understanding we have of our own Solar System to bear on understanding the properties of exoplanetary systems. It also gives us a broader view of solar-system formation and evolution than we can easily extract from the limited spatial and temporal perspective that we have on our home planetary system. This important new scientific endeavor is often referred to as “comparative exoplanetology.”
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