Direct Evidence for an Evolving Dust Cloud from an Exoplanet

Exoplanetology has progressed in several leaps. Initially, astronomers concentrated on simply discovering new planets outside our own Solar System. Having found over a thousand such bodies, and measured basic properties indicating their likely masses and for some of them, their sizes, the next step was to characterise them. This characterisation initially focused on the planets' atmospheres, as generally only the atmospheres emit and reflect light. Now, we are on the brink of the new era, where characterisation of exoplanetary surfaces and interiors could become feasible.

Characterisation of an exoplanetary atmosphere is possible in cases when the star-planet arrangement is such that the light originating from the host star occasionally travels through the planet's atmosphere - an event described as a planetary transit. If the atmosphere is sufficiently dense, and is extensive enough, its most abundant constituents can produce a unique spectroscopic imprint on the background starlight. From this the chemical composition of the atmosphere can be determined. A very similar technique could be successfully deployed to study surfaces of exoplanets, if planets can be found whose surfaces are at least partially "see through".

Prime objects for such observations are evaporating planets, whose outer layers get lifted into the surrounding space forming large, light absorbing/scattering clouds. The amount of light scattered by these clouds differs between shorter and longer wavelengths, depending on the size distributions of their constituent dust grains and their chemical compositions. This varying amount of scattering again leaves a unique imprint on the background light, which can be studied using multicolour photometry.

KIC 1255 b in transit. Artist's impression courtesy of Maciej Szyszko. [ PNG ]

Several potential candidates for such studies have been identified, including KIC 12557548, hypothesised to contain a transiting evaporating exoplanet which is so close to its host star that it completes an entire orbit (analogous to a year on Earth) in just 16 hours. KIC 12557548 b is thought to be a rocky planet more massive than Mercury, with a surface temperature of about 2100K. These properties cause a continuous loss of material through a wind, forming an extended tail of dust following the planet in its orbital path. Due to the self-limiting character of the mechanism driving the evaporation, the rate of dust ejection changes significantly between orbits, varying the amount of absorption observed during transits. This absorption produces a loss in the total observed light from the star, which can be measured as a depth (in per-cent) once every orbit when the planet and the entrained dust cloud passes across our line of sight to the star.

Astronomers observed KIC 12557548 on five nights in July 2013, using ULTRACAM on the William Herschel Telescope, to search for the colour dependence of the transit depth. To achieve the widest possible wavelength range with high enough signal-to-noise ratio, they used broad-band Sloan z', g' and u' filters. A clear transit signal was detected only on the first night (Figure 1). During the remaining four nights, the planet was in a quiescent period, with little or no dust cloud obscuring the star. As expected, the astronomers managed to observe differences in transit depths between the u', g' and z' filters on Night 1, with an increase in depth toward shorter wavelengths (Figures 1 & 2). This effect is consistent with extinction from the putative dust cloud surrounding the planet. Remaining nights allowed to constrain the size of the planet to less than 5.4 radii of Mercury.

Figure 1. Upper panel: z', g' and u' light curves of KIC 12557548 from 14 July 2013; best model fits are shown in red, binned data points are shown in black and raw data points are shown in orange and green for z' and g' bands respectively. Lower panel: g'/z' curve. Credit: Bochinski et al. (2015). [ PNG ]

After modelling Night 1 transit depths with the Cardelli, Clayton & Mathis (1989) interstellar extinction law, the extinction depths observed are broadly consistent with the wavelength-dependence of the reddening seen in the interstellar medium (Figure 2). This is strong independent and direct evidence in favour of the dust-cloud model for the transits. The color dependence of the transit depth is consistent with extinction due to dust with grain sizes between 0.25µm and 1µm. Analysis of a larger number of transit light curves potentially offers the opportunity to measure the composition of the grains. This is an unprecedented opportunity to learn the make-up of a rocky extrasolar planet. By doing that astronomers can find out how typical our Solar System is, and learn more about how Earth and other planets were formed. The team will be attempting to carry out such observations on the William Herschel Telescope in the summer of 2015.

Figure 2. The Cardelli, Clayton & Mathis (1989) interstellar extinction law fitted to our Night 1 transit depths. The best fit value was R_V = 5.3 (solid line). Extinction laws for values of R_V ranging between 2.50 and 9.00 are also shown. Credit: Bochinski et al. (2015). [ PNG ]

More information: 

Bochinski, Jakub J.; Haswell, Carole A.; Marsh, Tom R.; Dhillon, Vikram S. and Littlefair, Stuart P., 2015, "Direct evidence for an evolving dust cloud from the exoplanet KIC 12557548 b", ApJ Letters, 800, L21.

Contact: Javier Méndez  (Public Relations Officer)

Source:  Isaac Newton Group of Telescopes

 

NASA’s Kepler Detects Potential Evaporating Planet Candidate

The artist's concept depicts a comet-like tail of a possible disintegrating super Mercury-size planet candidate as it transits its parent star named KIC 12557548. At an orbital distance of only twice the diameter of its star, the surface temperature of the potential planet is estimated to be a sweltering 3,300 degrees Fahrenheit. At such a high temperature, the surface would melt and evaporate. The energy from the resulting wind would be enough to allow dust and gas to escape into space creating a trailing dusty effluence that intermittently blocks the starlight. Image credit: NASA/JPL-Caltech. Click here for full resolution.

A Curious Signature of a Potential Tiny World

The artist's animation depicts a possible disintegrating planet candidate as it orbits its star. Dust and gas ejected from the possible planet’s sweltering surface is theorized to form a comet-like tail of trailing material. The density of the tail can change dramatically, even over a single 15-hour orbit. Kepler finds planets by searching for the slight drop in brightness seen as they pass in front of their stars. Usually that drop is constant, but in this system, the variation seen from orbit to orbit hints at a dusty tail trailing a doomed world. Credit: NASA/JPL-Caltech

Astronomers may have detected evidence of a possible planet disintegrating under the searing heat of its host star located 1,500 light-years from Earth. Similar to a debris-trailing comet, the super Mercury-size planet candidate is theorized to fashion a dusty tail. But the tail won't last for long. Scientists calculate that, at the current rate of evaporation, the dusty world could be completely vaporized within 200 million years.

A research team led by Saul Rappaport, professor emeritus of physics at MIT, Boston, Mass., has identified an unusual light pattern emanating from a star named KIC 12557548 in the Kepler space telescope's field-of-view.

NASA's Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front, or transiting, their stars.

"The bizarre nature of the light output from this star with its precisely periodic transit-like features and highly variable depths exemplifies how Kepler is expanding the frontiers of science in unexpected ways," said Jon Jenkins, Kepler co-investigator at the SETI Institute in Mountain View, Calif. "This discovery pulls back the curtain of how science works in the face of surprising data."

Orbiting a star smaller and cooler than our sun, the planet candidate completes its orbit in less than 16 hours-- making it one of the shortest orbits ever detected. At an orbital distance of only twice the diameter of its star, the surface temperature of the planet is estimated to be a smoldering 3,300 degrees Fahrenheit.

Scientists hypothesize that the star-facing side of the potentially rocky inferno is an ocean of seething magma. The surface melts and evaporates at such high temperatures that the energy from the resulting wind is enough to allow dust and gas to escape into space. This dusty effluence trails behind the doomed companion as it disintegrates around the star.

Additional follow-up observations are needed to confirm the candidate as a planet. The finding is published in The Astrophysical Journal and is available for download at: http://arxiv.org/abs/1201.2662

For more details on the finding visit: http://web.mit.edu/newsoffice/2012/dusty-exoplanet-0517.html

For more information about the Kepler mission, visit: http://www.nasa.gov/kepler

Michele Johnson
Phone Number: (650) 604-6982
Ames Research Center, Moffett Field, Calif.