Science Nugget—Planet Hunting With the TMT: MIRES and NIRES
  Jay Elias
  NOAO New Initiatives Office

How common are planetary systems like our own? How frequently are the conditions for life present in such systems? Astronomers hope to answer—or come close to answering—these two fundamental questions during the next twenty years, using a variety of space—and ground-based observatories, of which TMT is an essential component.

In order to address the two questions, we need to answer a series of simpler questions:

• What determines the mass of a star?
  
• How and when do planetary systems form?
  
• What forms can planetary systems take?
  
• How frequently do terrestrial planets similar to Earth form and survive?
  
• How might the conditions for life be established in planetary systems, in particular on terrestrial planets?

The full TMT instrument suite contains several instruments that help answer several of these questions. In this article, we describe two such instruments: the Mid-Infrared Echelle Spectrograph/Imager (MIRES) and the Near-Infrared Echelle Spectrograph (NIRES). Future articles will describe the use of instruments such the as the Planet-Finding Imager (PFI).

In the last decade, astronomers have detected dozens of planets around nearby Sun-like stars, though the techniques used have limited detections, until recently, to giant planets like Jupiter. Most of these planetary systems are quite different from our own, with giant planets occupying the regions where Earth-like planets might lie. Are systems like ours rare, and if so, why? MIRES and NIRES can help answer this question by examining the formation of planetary systems from material around very young stars—the equivalent of the “solar nebula” from which our solar system formed 4.6 billion years ago.

To do this, these two instruments take advantage of the high spatial resolution provided by imaging at the diffraction limit with TMT (removing the blurring effects of the atmosphere with adaptive optics), and the high spectral resolution that can be achieved using the collecting area of the 30-meter diameter primary mirror. Figure 1 shows simulations of some representative cases (although MIRES will have much less spatial resolution than the simulations, it will be able to discriminate among these cases).

The left-hand image shows a disk which is just starting to form planetisimals (small protoplanets); there is emission from hot dust and gas throughout the disk. In the center image, a Jupiter-mass planet has formed, and it is restricting flow from the outer disk into the inner disk. This reduced accretion allows the formation of lower-mass planets; flux from the inner disk is reduced but still present, and accretion onto the large planet is also detectable. In the right-hand image, the planet is well above Jupiter mass, and is detectable directly. It also prevents material from reaching the inner disk, so no flux is seem from those regions.

Figure 1

At present, the gaps and disks can only be inferred indirectly from the spectra of these young stars, observed by facilities such as the Spitzer Space Telescope. MIRES and NIRES will also be able to observe the signatures of molecules and atoms in the disks, and measure orbital velocities as well; this allows one to determine the composition of the gas throughout the disk.

The second figure illustrates schematically where different molecules are found at increasing distances from a very young, Sun-like star and, thus, where different spectral features can be observed in a disk around such a star. The region shown covers distances where most known planets are found (from 0.1 to 10 times Earth’s distance from the Sun, known as an Astronomical Unit [AU] = 93 million miles).

The velocity profiles that might be observed at different distances from the star are also shown schematically. The innermost portions of the disk give rise to features that could be observed with NIRES; MIRES can observe transitions throughout the portions of the disk where planets might be found in a system like the solar system. The cooler material in the outermost portions of the disk will be observed by the Atacama Large Millimeter Array (ALMA).

Figure 2

MIRES will thus be able to measure the spatial distribution, velocity, and composition of material accreting into planets, and we will use these measurements to identify the locations and masses of massive planets, and the regions where lower-mass planets are formed.