Spitzer Space Telescope

Accurate measurement of exoplanet radius

Using data from NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist's conception. Image: NASA/JPL-Caltech

Image: Imaginative illustration of Kepler 93b’s diameter being measured. Credit: NASA/JPL-Caltech

Using both the Kepler and the Spitzer space telescopes, scientists from NASA have made the most precise measurement of an exoplanet’s radius yet. Kepler 93b, which orbits a dim star 300 ly away, has a diameter of 18,800 km, give or take 240 km. “The measurement is so precise that it’s literally like being able to measure the height of a six-foot tall person to within three-quarters of an inch – if that person were standing on Jupiter,” said Sarah Ballard, an astronomer at the University of Washington and lead author of a paper in The Astrophysical Journal that describes the findings, in a JPL statement.

Kepler 93b is a super-earth, a common class of planets in the Milky Way but missing in the Solar System. Super-earths weigh between the masses of Earth and Uranus. Scientists were able to its radius to within 240 km by first using the Kepler space telescope to record how much of starlight the exoplanet blocked when transiting across its face. Next, they used precise measurements of seismic waves moving within the star’s interior to calculate how much light it gave off and its radius. This technique falls within the field of astroseismology that has been used since the early 2000s. Astroseismic measurements are effective when the observatories have a long baseline, long observing time and high photometric precision.

The scientists were aided in their work by Kepler 93 being a cool dwarf star whose brightness varies less often and strongly enough to help constrain planetary transit and seismic measurements.

Then, the Spitzer space telescope used its Infrared Array Camera, or IRAC, to confirm that what Kepler was observing wasn’t a false-positive. It did this by using the fact that no matter which wavelength a transiting exoplanet is observed in, its transit depth will be the same. The transit depth is the ratio of the size of a planet’s disk to the star’s disk. So while Keplre measures this ratio in visible light, the IRAC will measure it in infrared light. To rule out a false-positive, the two measurements have to be the same.

The IRAC measurement was improved using a method developed in 2011, which checks how light falls on individual pixels in the camera. The scientists used Kepler 93b as a test bed, examining the exoplanet’s seven transits recorded between 2010 and 2011 in detail. Based on its mass – 3.8-times Earth’s – and radius, it was found to be made mostly of iron and rock, its biggest similarity to Earth. However, it orbits its star at a distance almost 19 times shorter than that between us and our Sun, making its surface too hot for life at 760 degrees Celsius.

Spitzer lost its coolant, and therefore the sensitivity of some of its instruments, in 2009. A telescope that measures heat coming in from various regions of the cosmos must have little heat of its own, which the cryogen ensured. Once it ran out, the temperature inside the telescope rose by 29 degrees Celsius, too warm for longer wavelength instruments but still cold enough for shorter wavelength ones like IRAC. The precision of the Kepler 93b measurement will give hope for future studies to understand why and how super-earths form, and instruments like IRAC will play an important role in that scenario.

References

Sarah Ballard et al., Kepler 93b: A terrestrial world measured to within 120 km, and a test case for a new Spitzer observing mode, 2014 ApJ 790 12 doi:10.1088/0004-637X/790/1/12 (pre-print)

JPL press release: The Most Precise Measurement of an Alien World’s Size, July 23, 2014

Spitzer has helped choose a near-Earth object the A.R.M. could bring nearer

From its perch up in space, Spitzer can use its heat-sensitive infrared vision to spy asteroids and get better estimates of their sizes.

This is what the author of a study that appeared in Astrophysical Journal Letters on June 19 said in a NASA press release about the space telescope. The Spitzer was used by a group of astronomers that authored the paper to study the dimensions and other physical properties of an asteroid named 2011 MD. They’ve found it to be suitable for NASA’s purpose, i.e. to bring a near-Earth object (NEO) into an orbit around the moon and study it – all by the 2020s. This elevation to suitability also makes 2011 MD the third such candidate NASA will consider as it ramps up the mission, dubbed the Asteroid Redirect Mission (ARM).

This image of asteroid 2011 MD was taken by NASA's Spitzer Space Telescope in Feb. 2014, over a period of 20 hours. The long observation, taken in infrared light, was needed to pick up the faint signature of the small asteroid (center of frame). The Spitzer observations helped narrow down the size of the space rock to roughly 20 feet (6 meters), making it one of a few candidates for NASA's proposed Asteroid Redirect Mission for which sizes are approximately known. — This image of asteroid 2011 MD was taken by NASA’s Spitzer Space Telescope in Feb. 2014, over a period of 20 hours. The long observation, taken in infrared light, was needed to pick up the faint signature of the small asteroid (center of frame). The Spitzer observations helped narrow down the size of the space rock to roughly 20 feet (6 meters), making it one of a few candidates for NASA’s proposed Asteroid Redirect Mission for which sizes are approximately known. Image: NASA/JPL-Caltech/Northern Arizona University/SAO

Why was Spitzer used? From the release:

Prior to the Spitzer study, the size of 2011 MD was only very roughly known. It had been observed in visible light, but an asteroid’s size cannot be determined solely from visible-light measurements. In visible light alone, for example, a white snowball in space could look just as bright as a dark mountain of cosmic rock. The objects may differ in size but reflect the same amount of sunlight, appearing equally bright.

The advantage that infrared light presents, on the other hand, is that it reveals the body’s temperature, mass and density. Subsequently, the study’s authors were able to conclude that 2011 MD is lighter than asteroids usually are, and is possible two-thirds hollow. This, they think, could be because it is actually a collection of rocks or is one rock surrounded by debris. One more thing about this new candidate for ARM is its odd, oblong shape.

The team says the small asteroids probably formed as a result of collisions between larger asteroids, but they do not understand how their unusual structures could have come about. They plan to use Spitzer in the future to study more of the tiny asteroids, both as possible targets for asteroid space missions, and for a better understanding of the many asteroid denizens making up our solar system.

Knowing the size of the NEO to bring closer is important because it will help NASA plan the “how” of the mission. In another press release yesterday, the space agency said it was awarding $4.9 million to 18 proposals each of which described a method to execute the ARM, over a period of six months. NASA started accepting these proposals in March this year and reportedly received 108. Two names quickly jump out from among the proposals:

Deep Space Industries, which announced in January 2013 that it plans to scout for a near-Earth object, mine a small sample from it, and return that to Earth by 2016. The press release states that, through the ARM, DSI wants to “examine public-private partnership approaches”.
Planetary Society, which wants to put bacteria on the asteroid retrieval vehicle to “transport extremophiles through deep space and return them to Earth to test panspermia and astrobiology.”

The 2011 MD press release is available here, and the one about the proposals, here.