Jupiter's Moons Remain Slightly Illuminated, Even in Eclipse

Astronomers using the Subaru Telescope and Hubble Space Telescope have found that Jupiter's Galilean satellites (Io, Europa, Ganymede, and Callisto) remain slightly bright (up to one millionth of their normal state) even when in the Jovian shadow and not directly illuminated by the Sun (Figure 1). The effect is particularly pronounced for Ganymede and Callisto. The finding was made by researchers at Tohoku University, Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency (ISAS/JAXA), National Astronomical Observatory of Japan (NAOJ), and elsewhere.

Figure 1: Images of Ganymede and Callisto while eclipsed by Jupiter obtained during their eclipse. Top left is Ganymede observed through Subaru Telescope, top right is Ganymede through Hubble Space Telescope, bottom left is Callisto from Subaru Telescope, respectively. Each frame is 4 arcsec x 4 arcsec, and the black circle indicates the apparent diameter of the object. A short movie linked here shows the Europa's eclipse as it goes into the shadow of Jupiter. From the top of the video is Europa, Ganymede, and Jupiter, respectively. (Credit: NAOJ/JAXA/Tohoku University)

The mechanism of this phenomenon is still under investigation, but the researchers suggest that indirect forward scattering of sunlight by hazes in the upper Jovian atmosphere could be the reason for the illumination. This effect is similar to the one that causes Earth's moon to look red during a total lunar eclipse.

The type of continuous observations of the Galilean satellites in eclipse made by the Japanese team also provides a much better basis for studying the hazes in Jupiter's atmosphere, which are difficult to study otherwise (Note 1). In addition, this detailed study method for a planetary atmosphere will provide new insights about the atmospheres of exoplanets, which are only beginning to be studied. 

Dr. Kohji Tsumura (FRIS, Tohoku University), the PI on the project, explained that this unexpected finding is really the outcome of attempts to measure diffuse light from the distant universe. "It is a serendipitous discovery made as a by-product of a cosmological study," he said. "It is very interesting that it provides us a new method to investigate the atmosphere of Jupiter and of exoplanets. I will keep studying from nearby space (the solar system and exoplanets) out to the farthest universe through this project."

The research team started its observations using IRCS and AO188 on Subaru Telescope in February of 2012. The idea was to detect the diffuse light from the most distant parts of the universe. To do this, team members planned to use the Galilean satellites in eclipse as "occulters" to block distant background emissions. This would allow an extremely accurate separation of the background light from the very bright foreground radiation from our solar system (known as the zodiacal light).

The team assumed that the Galilean satellites would be "dark" while in Jupiter's shadow, and the difference in brightness[??] between the dark satellite as an occulter and its surrounding sky would allow the team to determine the still-unknown level of background emission from the distant universe. Instead, they found an unexpected surprise: Ganymede and Callisto were still somewhat "bright" (illuminated) even when eclipsed (relative to the expected level of near-zero). Their eclipsed luminosity was one millionth of their un-eclipsed brightness, which is low enough that this phenomenon has been undetected until now (Figure 2).

Figure 2: Schematic drawing of how to measure the background light. By measuring the difference between the light from the eclipsed satellite and the surrounding sky data, one would obtain the background light information, if the eclipsed satellite is completely dark. It turned out that there is still a faint light reflected off from the satellite during the eclipse. (Credit: NAOJ/JAXA/Tohoku University)

To understand why the Galileans remain ever-so-slightly bright even when they're in eclipse, the project team of astronomers and planetary scientists considered several theories based on their multi-band observational data, including data from Spitzer Space Telescope. The most plausible is that the Galilean satellites are still illuminated during eclipse by sunlight that is scattered by hazes in the Jovian upper atmosphere. By comparison, the sunlight refracted in the atmosphere does not contribute to the illumination during the eclipse.

Although Jupiter is a familiar planet, there are many unresolved issues about its atmosphere. One example is the origin of the cloud particles composing Jupiter's banded appearance. The cloud particles are assumed to grow from tiny particles called aerosols or hazes. Researchers expect that those hazes form somewhere in the upper part of Jupiter's atmosphere, which is very difficult to observe (Figure 3). The unexpected discovery of haze-induced brightening of the Galileans provides a new way to study the mysterious part of Jupiter's atmosphere. In addition, since astronomers usually observe the planets in our solar system by reflected sunlight, one of the unique aspects of these new observations at Jupiter is that observers can precisely measure the transmitted sunlight through the planetary atmosphere (Note 2).

Figure 3: A schematic image of the model to show that the Galilean satellites eclipsed in Jovian shadow are illuminated by scattered sunlight by the haze in the Jovian upper atmosphere. The size and the distance of the satellites are not to the scale. This process is similar to one that causes red color of the Earth's Moon during its total eclipse. (Credit: NAOJ/JAXA/Tohoku University/NASA)

This new method of studying the upper atmosphere of Jupiter via transmitted sunlight provides a basis for the study of other planetary systems. Exoplanet discoveries now occur quite regularly and atmospheres around some of them have been investigated using "transit observations" (when the exoplanet passes between us and the host star, resulting in the star becoming slightly dimmer). In such observations, some characteristics of the exoplanet's atmosphere are revealed as host starlight passes through it. This is the same situation seen with Jupiter and its Galilean satellites, and makes studies of transmitted sunlight of the planets in our solar system essential for comparison.

The observations for this project were very challenging because the Galilean satellites (while eclipsed) are extremely faint and they are located next to the incredibly bright disk of Jupiter. In addition, the eclipses only happen at very specific times, and Jupiter and the satellites are continuously in motion during the observations. The complexity of the situation requires the observation procedure to be much more sophisticated. This new discovery required thorough preparations by the project team and conscientious support by the operations staff. (Figure 4)

Figure 4: Three-color (JHK) image of Jupiter and Ganymede obtained by Subaru Telescope. Because Ganymede moves with respect to Jupiter during the observations, it appears as three separately colored dots. Image taken at around 5 am, July 27, 2012 in Hawaii Time. Blue color is for J band (1.3 micrometer), green is for H band (1.6 micrometer), and red is for K band (2.2 micrometer), respectively. (Credit: NAOJ/JAXA/Tohoku University)

The results of this study will be published in The Astrophysical Journal in its July 10, 2014 issue (Tsumura et al. 2014 arXiv: 1405.5280). This research is supported by Japan Society for the Promotion of Science, KAKENHI (#24111717, #26800112), and NASA through a grant from Space Telescope Science Institute and Jet Propulsion Laboratory in U. S. A.

Notes:

1. Observations of Jupiter's upper atmosphere were conducted by the Galileo spacecraft, and by "occultation" of microwaves from field stars or spacecraft located behind Jupiter. However, opportunities to observe these events are rare and limited, thus the observations of the Galilean satellite eclipses are a unique method to study Jupiter's upper atmosphere.
2. Transmitted light through the planetary atmosphere was observed by the occultation methods described in (*1) and the Venus transit. However, they are very rare events, too.

Paper information:

To appear in the July 10, 2014 issue of the Astrophysical Journal, Volume 789.
"Near-infrared Brightness of the Galilean Satellites Eclipsed in Jovian Shadow: A New Technique to Investigate Jovian Upper Atmosphere"
K. Tsumura (1,2), K. Arimatsu (2,3), E. Egami (4), Y. Hayano (5), C. Honda (6), J. Kimura (7), K. Kuramoto (8), S. Matsuura (2), Y. Minowa (5), K. Nakajima (9), T. Nakamoto (10), M. Shirahata (2, 11), J. Surace (12), Y. Takahash i(8), and T. Wada (2)

¹Frontier Research Institute for Interdisciplinary Science, Tohoku University, Japan

²Department of Space Astronomy and Astrophysics, Institute of Space and Astronoutical Science, Japan Aerospace Exploration Agency, Japan

³Department of Astronomy, Graduate School of Science, The University of Tokyo, Japan

⁴Department of Astronomy, Arizona University, U. S. A.

⁵Subaru Telescope, National Astronomical Observatory of Japan, U. S. A.

⁶Research Center for Advanced Information Science and Technology, Aizu Research Cluster for Space Science, The University of Aizu, Japan

⁷Earth-Life Science Institute, Tokyo Institute of Technology, Japan

⁸Department of Cosmosciences, Graduate School of Science, Hokkaido University, Japan

⁹Department of Earth and Planetary Sciences, Kyushu University, Japan

¹⁰Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology, Japan

¹¹National Astronomical Observatory of Japan, Japan

¹²Spitzer Science Center, California Institute of Technology, U. S. A.

Source: Subaru Telescope

First Directly Imaged Planet Confirmed Around Sun-like-Star

Figure 1 First released in September of 2008: Gemini adaptive optics image of 1RXS J160929.1-210524 and its ~8 Jupiter-mass companion (within red circle). This image is a composite of J-, H- and K-band near-infrared images. All images obtained with the Gemini Altair adaptive optics system and the Near-Infrared Imager (NIRI) on the Gemini North telescope. Photo Credit: Gemini Observatory. JPG 89.39 KB | TIFF 18.7 MB

Credit: Gemini Observatory/AURA/David Lafrenière (University of Montreal),Ray Jayawardhana (University of Toronto), and Marten van Kerkwijk (University of Toronto)

Figure 2. New images of 1RXS J160929.1-210524 at 3.05 and 3.8 microns left and right respectively. Images obtained using the Gemini Near Infrared Imager (NIRI) with the Altair adaptive optics system. These data were used to determine a better estimate the planet's mass.

Credit: Gemini Observatory/AURA/David Lafrenière (University of Montreal),Ray Jayawardhana (University of Toronto), and Marten van Kerkwijk (University of Toronto)

Figure 3. Proper motion plots from Gemini observations of 1RXS J160929.1-210524 confirming that the star and planet are a bound system.

Credit: Gemini Observatory/AURA/David Lafrenière (University of Montreal),Ray Jayawardhana (University of Toronto), and Marten van Kerkwijk (University of Toronto)

Figure 4. (top) Comparison of the planet spectrum (black) with a slightly warmer, more massive free floating brown dwarf in the Upper Scorpius association (red, from Lodieu et al. 2008). (bottom) Comparison with a model spectrum of temperature 1800 K and low surface gravity (red, from DRIFT PHOENIX models, Witte et al. 2009, Helling et al. 2008).

A planet only about eight times the mass of Jupiter has been confirmed orbiting a Sun-like star at over 300 times farther from the star than the Earth is from our Sun. The newly confirmed planet is the least massive planet known to orbit at such a great distance from its host star. The discovery utilized high-resolution adaptive optics technology at the Gemini Observatory to take direct images and spectra of the planet.

First reported in September 2008 by a team led by David Lafrenière (then at the University of Toronto, now at the University of Montreal and Center for Research in Astrophysics of Quebec), the suspected planetary system required further observations over time to confirm that the planet and star were indeed moving through space together. “Back in 2008 what we knew for sure was that there was this young planetary mass object sitting right next to a young Sun-like star on the sky,” says Lafrenière. The extremely close proximity of the two objects strongly suggested that they were associated with each other but it was still possible (but unlikely) that they were unrelated and only aligned by chance in the sky. According to Lafrenière, “Our new observations rule out this chance alignment possibility, and thus confirms that the planet and the star are related to each other.”

With this confirmation by Lafrenière and colleagues, the system, known as 1RXS J160929.1-210524 (or 1RXS 1609 for short), provides scientists with a unique specimen that challenges planetary formation theories due to its extreme separation from the star. "The unlikely locale of this alien world could be telling us that nature has more than one way of making planets," says co-author Ray Jayawardhana of the University of Toronto. "Or, it could be hinting at a violent youth when close encounters between newborn planets hurl some siblings out to the hinterlands," he adds.

With its initial detection by the team using the Gemini Observatory in April of 2008 this object became the first likely planet known to orbit a sun-like star that was revealed by direct imaging. At the time of its discovery the team also obtained a spectrum of the planet and was able to determine many of its characteristics, which are confirmed in this new work. “In retrospect, this makes our initial data the first spectrum of a confirmed exoplanet ever!” says Lafrenière. The spectrum shows absorption features due to water vapor, carbon monoxide, and molecular hydrogen in the planet’s atmosphere.

Since the initial observations several other worlds have been discovered using direct imaging, including a system of three planets around the star HR 8799 also discovered with Gemini. However, the planets around HR 8799 orbit much closer to their host star.

The team’s recent work on 1RXS 1609 also verified that no additional large planets (between 1-8 Jupiter masses) are present in the system closer to the star. Future observations may shed light on the origin of this mysterious far-out planet. In particular, in a few years, it should be to possible to detect a slight difference in motion between the planet and its star due to their mutual orbit. Co-author Marten van Kerkwijk (University of Toronto) notes that the difference will be “very small,” since the fastest possible orbital period is more than one thousand years. But he adds that by using Gemini it should be possible to measure a very precise velocity of the planet relative to its host. This will show whether the planet is likely on a roughly circular orbit, as would be expected if it really formed far from its host star, or whether it is in a very non-circular or even unbound orbit, as could be the case if it formed closer to its star, but was kicked out in a close encounter with another planet.

The host star is located about 500 light-years away in a group of young stars called the Upper Scorpius association that formed about five million years ago. The original survey studied more than 85 stars in this association. The planet has an estimated temperature of about 1800 Kelvin (about 1500 degrees Celsius) and is much hotter than Jupiter, which has a atmospheric cloud-top temperature of about 160 Kelvin (-110 degrees Celsius). The host star has an estimated mass of about 85% that of our Sun. The young age of the system explains the high temperature of the planet. The contraction of the planet under its own gravity during its formation quickly raised its temperature to thousands of degrees. Once this contraction phase is over, the planet slowly cools down by radiating infrared light. In billions of years, the planet will eventually reach a temperature similar to that of Jupiter.

The observations used the Near-Infrared Imager (NIRI) and the Altair adaptive optics system on the Gemini North telescope. Adaptive optics allows scientists to remove much of the distortions caused by our atmosphere and dramatically sharpen views of space. “Without adaptive optics, we would simply have been unable to see this planet,” says Lafrenière. “The atmosphere blurs the image of a star so much that it extends over and is much brighter than the image of a faint planet around it, rendering the planet undetectable. Adaptive optics removes this blurring and provides a better view of faint objects very close to stars.”

The result has been accepted for publication (see preprint here) in an upcoming issue of The Astrophysical Journal.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawai'i (Gemini North) and the other telescope at Cerro Pachón in northern Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Science and Technology Facilities Council (STFC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.

Science Contacts:

Dr. David Lafrenière
University of Montreal
Email: david@astro.umontreal.ca
Phone (office): (514) 343-6111 (Ext. 3190)

Prof. Ray Jayawardhana
University of Toronto
Email: rayjay@astro.utoronto.ca
Phone (office): (416) 946-7291

Media Contact:

Peter Michaud
Gemini Observatory, Hilo, HI
Email: pmichaud@gemini.edu
Cell: (808) 936-6643
Desk: (808) 974-2510