The search for planets outside our solar system is an underreported but active field. Dozens of powerful instruments have been designed to detect these faint heavenly bodies in unique ways. Whether ground-based or in orbit, they face numerous challenges. The Gemini Planet Imager (GPI) opened in November last year and is the most recent addition to this exclusive group of space investigators.
The GPI is the world’s most advanced instrument for directly observing light coming from Jupiter-sized, or Jovian, planets. Located in Chile, it is the culmination of a decade of work among many institutions, including UC Santa Cruz, the Lawrence Livermore National Observatory and UCLA. GPI is designed to observe the atmospheres of large young planets within tens of light years away and take their spectra, which reveals the planets’ composition.
“Newly-formed planets are very warm, and the Gemini Planet Imager looks in the infrared to detect their light,” said Jeffrey Chilcote, a graduate student at UCLA and a contributor to the GPI project.
Hot solid objects emit light with a continuous spectrum across many wavelengths. Hot gases emit light at only specific wavelengths. Consequently, hot solid objects surrounded by cooler gasses will show a near-continuous spectrum with dark lines corresponding to the emission lines of the gasses. By comparing the absorption lines of the object with emission spectra of known gases, the chemical composition of objects can be determined.
However, several problems stand in the way.
“The main problem for direct imaging is atmospheric seeing (twinkling of starlight) … This basically distorts the image and makes it look larger by moving the image around so we do not have very good detail resolution,” said Patricia Boeshaar, a UC Davis astrophysics professor.
Furthermore, the brightness of the host star, which is often seven or eight magnitudes higher than the target planet, make observing the planet difficult.
“[It’s like] trying to see a firefly on the rim of a searchlight,” Boeshaar said.
The GPI contains several components to overcome these obstacles. Its adaptive optics system, an essential organ in any serious ground-based space observing endeavor, corrects the light coming through our atmosphere.
A large deformable mirror takes the distorted light and reflects it as corrected waves in a process similar to how a carnival mirror changes how you look. The GPI’s coronagraph blocks out the light coming from the host star so that its planets may be more easily visible. The sky is imaged onto a mirror with a small hole: the desired light is reflected, but the unwanted light from the star goes through the hole. While imperfect, these corrections allow enough legroom for valuable data to be collected.
The real goal of GPI and any exoplanet search activity is to uncover clues about the formation of our universe. What causes planets to form around stars? How common are planets in solar systems?
“We have a couple of questions about Jovian planet formation mechanisms. Do they form by core accretion, where the solar system coagulates but eventually builds up gas, versus planet formation like gravitational collapse?” said Michael Fitzgerald, astrophysics professor at UCLA.
In core accretion — the current prevailing theory — a core with a large mass forms by numerous collisions between smaller bodies. Hydrogen and helium eventually envelope the core until the supply (usually leftovers from star formation) is exhausted. In gravitational instability, the self gravity of a massive gas disk collapses into fragments, becoming giant planets. No solid core is required.
” There are also questions about system evolution. We have inferred that the orbits of our giant planets are not where they were long ago,” Fitzgerald said.
This is inferred by the distribution of small bodies in the solar system, like the asteroid belt and Kuiper belt. Their orbital properties indicate a migration.
GPI is set to conduct a 600 star survey to observe their planets this year. How many questions it can answer about our universe remains to be seen.
