The Spitzer Space Telescope Has Been Reborn as an Exoplanet Hunter

It’s been months since NASA’s Kepler Space Telescope, the exoplanet hunter, lost its ability to lock on to distant stars and find planets orbiting around them. But it looks like NASA’s found a replacement telescope—the Spitzer Space Telescope—to keep the exoplanet hunt going while we wait for the James Webb Space Telescope to launch.

The Spitzer is going on ten years old, and has gotten an upgrade that will let it study distant stars for signs of life-bearing planets. The engineers and scientists who built Spitzer did not have exoplanet hunting in mind for their telescope. Far from it. Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena said that when Spitzer launched in 2003, the idea of using it to study exoplanets was so crazy that no one even considered it.

Spitzer was the final mission in NASA’s Great Observatories Program, a program that comprised four space telescopes all designed to look at the Universe and detect different types of light. Hubble is the visible light telescope, the Compton Gamma-Ray Observatory looks at gamma-rays, and the Chandra X-Ray Observatory detects x-rays. Spitzer was designed to last up to two-and-a-half years, during which it would detect infrared radiation, a long wavelength of light that occurs primarily in the form of heat radiation.

The telescope has done just that, though it’s far outlived its primary mission, using its two main components: the Cryogenic Telescope Assembly that contains the 33 foot telescope and the related science instruments, and the spacecraft that controls the telescope. It’s the largest infrared telescope ever launched into space, and its highly sensitive instruments have allowed astronomers to look into previously unseen regions of the Universe. Spitzer has shown us dusty stellar nurseries, the centres of galaxies, and newly forming planetary systems.

Spitzer was also able to see exoplanets, even if it wasn’t designed to find these far-off worlds. It can see the tell-tale dimming of a star when a planet crosses in front of it. It can also see exoplanets directly since exoplanets emit infrared light. Spitzer has the incredible capability to detect and study the infrared light coming from a planet and use it to determine the planet’s atmospheric composition. And as an exoplanet orbits its star, it shows different regions to Spitzer’s camera, divulging information on the planet’s climate. As the planet’s light dims when it goes behind the star, Spitzer can learn about its global temperature.

But Spitzer had to learn how to look at exoplanets; this capability was made possible by the telescope reaching an unprecedented level of sensitivity well beyond its original design specifications.

Like all missions, Spitzer’s design and science payload was finalized and frozen well before launch, in this case in 1996. Part of its design was to keep firm control over temperature variations and an excellent star-targeting pointing system. These two elements are what have allowed Spitzer to become a modern day exoplanet hunter. It can lock onto a distant star and very precisely measure the infrared light coming from the exoplanet.

But the team behind the telescope still had to make a few adjustments. Scientists observed a slight wobble when the telescope pointed at a target star, and the camera exhibited small brightness fluctuations.

To address the wobble, scientists first changed the spacecraft’s onboard heater cycle; they found it was turning on at inopportune times and causing the camera to move. They then went a step further and repurposed Spitzer's Pointing Control Reference Sensor "Peak-Up" camera. Designed to gather infrared light precisely into a spectrometer for routine calibrations, this camera had far fewer issues with wobbling. So engineers applied the Peak-Up to the infrared camera observations, which has allowed astronomers to place stars precisely on the center of a camera pixel.

These two fixes, along with the foresight of launching the mission with backup temperature control systems, have more than doubled Spitzer's stability and targeting. The telescope now has a second life as an exquisitely sensitive machine for measuring exoplanets.