Tag: Kepler space telescope

Astronomers strike gold with find of unique star-system only 150 ly away

After the Kepler Space Telescope lost two if its four reaction wheels, it was unable to point accurately enough for long observations. A retooled mission, dubbed K2, is still able to obtain images of transiting planets by looking along the plane of the galaxy, or the ecliptic. NASA image.
After the Kepler Space Telescope lost two if its four reaction wheels, it was unable to point accurately enough for long observations. A retooled mission, dubbed K2, is still able to obtain images of transiting planets by looking along the plane of the galaxy, or the ecliptic. NASA image.

Even after the loss of a critical stabilization system on-board, the NASA Kepler space telescope has made an important discovery. Scientists from the University of California have used the telescope to find a nearby star-system comprising a cool red M-dwarf and three planets slightly larger than Earth orbiting it. The find’s significance stems from various reasons, not the least of which is that this system is only 150 light-years away, close enough for astronomers to make direct observations of the star and the planets’ atmospheres*.

The three planets’ radii are 2.1-, 1.7- and 1.5-times that of Earth. The outermost planet, with the radius 1.5 RE, is in fact on the inner edge of the system’s habitability zone and receives 1.4-times the light that Earth does from our Sun. Moreover, follow-up observations made from the Automated Planet Finder telescope, California, and the Keck Telescope, Hawaii, indicate that the planets’ surfaces are cool and not scorched as exoplanets’ surfaces have often been found to be. The ‘lukewarm’ temperature is a sign that the planets are fully-formed.

For these reasons and others, finding this star-system has been like striking gold for astronomers. Its proximity permits them to closely monitor its evolution than if it had been farther away. The red M-dwarf at its center – designated EPIC 201367065 – is not too bright or its electromagnetic flux would have ‘bleached’ out observations; its moderate emission also means the planets’ surfaces aren’t scorched. The almost Earth-sized planet just about in the habitable zone means they can study if its surface conditions are conducive to life (A recent analysis concluded that one in five Sun-like stars in the Milky Way hosts an Earth-sized planet in the habitable zone, which means there are 40 billion such planets in our galaxy alone). Making matters easier overall is the proximity of the system itself, which also means investigations can be more detailed for the same resources.

One such detail that has not been explored with any great precision among farther exoplanets is composition. With sizes in the 1.5-2.1 RE range, the study’s authors think “they may span the gap between rock-dominated ‘Earths’/’super-Earths’ and low-density ‘sub-Neptunes’ with considerable volatile content”. Compositional analyses are important to understand what kind of planets can form under what conditions and how their orbits could have migrated within the system before attaining equilibrium. Additionally, they could also help astronomers understand why there are no planets heavier than Earth but lighter than Neptune in our Solar System.

Anyway, the next course of action will be to use the Hubble space telescope to compose a spectroscopic map of the outermost planet’s atmosphere. Many exoplanets that possess atmospheres also possess hydrogen-rich atmospheres, with no hints of the oxygen and nitrogen that have been able to support life on Earth. If the outermost planet’s atmosphere is also dominated by hydrogen, then the gas’s presence will show up in Hubble’s measurements. As the study’s lead author Ian Crossfield, from the Lunar & Planetary Laboratory at the University of Arizona, noted, the presence of large quantities of hydrogen doesn’t preclude life but only life as we’ve known it on Earth.

The study’s paper was uploaded to the arXiv pre-print server on January 15 and has been submitted to the Astrophysical Journal.

*Although the ten closest star-systems that have exoplanets are within 20 ly of the Sun.

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.

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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