Science Nugget—TMT's Planet Formation Instrument
  Bruce Macintosh, Lawrence Livermore National Laboratory &
  James Graham, UC Berkeley

In the closing years of the 20th century, humankind began its exploration of the planetary systems in the solar neighborhood. Precision radial velocity measurements have now yielded the discovery of over 160 planets. Direct imaging of these planets—as opposed to detection of the effects of orbital motion on their parent star—is now feasible, and the first young planet in a wide orbit may have been detected using adaptive optics systems.

Gemini and the VLT are building the first generation of high contrast “extreme” adaptive optics (ExAO) systems: the Gemini Planet Imager and SPHERE. These systems will combine advanced adaptive optics with a device called a coronagraph (named after experiments to study the corona of the Sun), which blocks diffracted light from a star to make nearby objects visible.

The systems will make the first surveys of the outer regions of solar systems by detecting the self-luminous radiation of young planets (1-10 Jupiter masses, with an age from 10-1,000 million years). However, a typical coronagraph blocks out everything closer to the star than 3-5 times the diffraction limit of the telescope, referred to as the “inner working distance.”

ExAO on 8-meter telescopes systems will only be able to see planets at separations greater than 0.1 arcseconds—they can’t see close enough to the host stars to image Doppler-detected planets or other mature planets in reflected light. They cannot reach the equivalent of the orbit of Jupiter in the relatively distant young clusters and associations where planets are likely to still be forming.

Because of their short lifetimes relative to the Galactic star formation rate, young planet-forming systems are rare and found in significant numbers only in distant star-forming clouds (greater than 150 parsecs (pc), or about 500 light-years from Earth). The inner working distance required to study planet formation in situ is therefore about 35 milliarcseconds (5 AU at 150 pc), within reach for a 30-meter telescope.

Using a facility dubbed the Planet Formation Instrument (PFI), TMT can see into the “snow line” of these young systems, where temperatures drop low enough for the icy cores of planets like Jupiter to form. TMT can probe solar system-like scales for stars that span the whole age range of the planet formation process, and thus could be the first facility to witness the formation of new planets directly.

Likewise, to detect nearby mature planets in reflected light, a comparable angular resolution will be needed (30 milliarcseconds = 0.3 AU at 10 pc). PFI will use the nearly four-fold improved angular resolution of TMT to peer into the inner solar systems of planet-bearing stars to yield a unified sample of planets with known Keplerian orbital elements and atmospheric properties. TMT will thus also be the first facility to be able to directly detect a sizable number of reflected-light Jovian planets.

To achieve this sensitivity, even on a 30-meter telescope, requires an extremely advanced instrument. A team consisting of the Lawrence Livermore National Laboratory, UC Berkeley, JPL, and University of Montreal completed a feasibility study for PFI. The design combines multiple Micro-Electro-Mechanical-System (MEMS) deformable mirrors, including tens of thousands of actuators lithographically etched out of silicon, with advanced infrared and interferometric wavefront sensing to remove optical aberrations from the atmosphere or telescope.

Ultraprecise mirror surfaces—smooth to within one nanometer—and precise calibration reduce self-inflicted quasi-static optical errors that could otherwise hide a planet. Finally, light from the TMT aperture is divided and destructively interfered against itself in a kind of single-telescope interferometer to cancel the effects of diffraction. Interestingly, TMT’s segmented mirror has almost no effect on its ability to detect planets. The gaps between TMT mirror segments are so small and regular that they scatter light out into a wide grid of faint points, rather than concentrating light where planets might be found.

The combined system can see planets less than 30 miliarcseconds away from bright stars. The unique combination of angular resolution—greater even than proposed planet-finding spacecraft—and sensitivity will thus enable direct images and spectra to be obtained for both young and old planets (figure 1), leading to the first complete survey of the origin and evolution of other planetary systems.

Figure 1: This graph plots the brightness (contrast) ratio between star-to-planet versus their separation distance for a Monte Carlo simulation of a variety of targets in the solar neighborhood. Blue dots are rocky planets, beyond the reach of even TMT. Black dots are mature Jovian planets reflecting sunlight. Green dots are self-luminuous Jovian planets, typically those with masses of 3-10 Jupiter masses and ages of less than one billion years. Red dots are extremely young planets, recently formed or still accreting, such as in the Taurus star-forming region. The expected sensitivities of PFI and the Gemini Planet Imager for a bright (4th magnitude) target are overlaid.