Gathering detailed information on exoplanets is extremely difficult. The light from their host star overwhelms the light from the exoplanet, making it difficult for telescopes to see them. But now a team using cutting-edge technology at the Keck Observatory has taken a big leap in exoplanet observation and has detected water in the atmosphere of a planet 179 light years away.
Located about 129 light years from Earth in the direction of the Pegasus constellation is the relatively young star system of HR 8799. Beginning in 2008, four orbiting exoplanets were discovered in this system which – alongside the exoplanet Formalhaut b – were the very first to be confirmed using the direct imaging technique. And over time, astronomer have come to believe that these four planets are in resonance with each other.
In this case, the four planets orbit their star with a 1:2:4:8 resonance, meaning that each planet’s orbital period is in a nearly precise ratio with the others in the system. This is a relatively unique phenomena, one which inspired a Jason Wang – a graduate student from the Berkeley arm of the NASA-sponsored Nexus for Exoplanet System Science (NExSS) – to produce a video that illustrates their orbital dance.
Using images obtained by the W.M. Keck Observatory over a seven year period, Wang’s video provides a glimpse of these four exoplanets in motion. As you can see below, the central star is blacked out so that the light reflecting off of its planets can be seen. And while it does not show the planets completing a full orbital period (which would take decades and even centuries) it beautifully illustrates the resonance that exists between the star’s four planets.
As Jason Wang told Universe Today via email:
“The data was obtained over 7 years from one of the 10 meter Keck telescopes by a team of astronomers (Christian Marois, Quinn Konopacky, Bruce Macintosh, Travis Barman, and Ben Zuckerman). Christian reduced each of the 7 epochs of data, to make 7 frames of data. I then made a movie by using a motion interpolation to interpolate those 7 frames into 100 frames to get a smooth video so that it’s not choppy (as if we could observe them every month from Earth).”
The images of the four exoplanets were originally captured by Dr. Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics. It was in 2008 that Marois and his colleagues discovered the first three of HR 8799’s planets – HR 8799 b, c and d – using direct imaging technique. At around the same time, a team from UC Berkeley announced the discovery of Fomalhaut b, also using direct imaging.
These planets were all determined to be gas giants of similar size and mass, with between 1.2 and 1.3 times the size of Jupiter, and 7 to 10 times its mass. At the time of their discovery, HR 8799 d was believed to be the closest planet to its star, at a distance of about 27 Astronomical Units (AUs) – while the other two orbit at distances of about 42 and 68 AUs, respectively.
It was only afterwards that the team realized the planets had already been observed in 1998. Back then, the Hubble Space Telescope’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) had obtained light from the system that indicated the presence of planets. However, this was not made clear until after a newly-developed image-processing technique had been installed. Hence, the “pre-discovery” went unnoticed.
Further observations in 2009 and 2010 revealed the existence of fourth planet – HR 8799 e – which had an orbit placing it inside the other three. Even so, this planet is fifteen times farther from its star than the Earth is from the Sun, which results in an orbital period of about 18,000 days (49 years). The others take around 112, 225, and 450 years (respectively) to complete an orbit of HR 8799.
Ultimately, Wang decided to produce the video (which was not his first), to illustrate how exciting the search for exoplanets can be. As he put it:
“I had written this motion interpolation algorithm for another exoplanet system, Beta Pictoris b, where we see one planet on an edge-on orbit looking like it’s diving into its star (it’s actually just circling in front of it). We wanted to do the same thing for HR 8799 to bring this system to life and share our excitement in directly imaging exoplanets. I think it’s quite amazing that we have the technology to watch other worlds orbit other stars.”
In addition, the video draws attention to a star system that presents some unique opportunities for exoplanet research. Since HR 8799 was the first multi-planetary system to be directly-imaged means that astronomers can directly observe the orbits of the four planets, observe their dynamical interactions, and determine how they came to their present-day configuration.
Astronomers will also be able to take spectra of these planet’s atmospheres to study their composition, and compare this to our own Solar System’s gas giants. And since the system is really quite young (just 40 million years old), it can tell us much about the planet-formation process. Last, but not least, their wide orbits (a necessity given their size) could mean the system is less than stable.
In the future, according to Wang, astronomers will be watching to see if any planets get ejected from the system. I don’t know about you, but I would consider a video that illustrates one of HR 8799’s gas giants getting booted out of its system would be pretty inspiring too!
Further Reading: NASA
This year marks the 20th anniversary of 51 Peg b, the first exoplanet detected around a Sun-like star. And although the number of sheer detections in the years since have been remarkable, it’s also remarkable how little we still know about these alien worlds, save for their distances from their host stars, their radii, and sometimes their masses.
But the ability to directly image these worlds provides the opportunity to change all that. “It’s the tip of the iceberg,” said Marshall Perrin from the Space Telescope Science Institute in a press conference at the American Astronomical Society’s meeting earlier today. “In the long run, we think that imaging offers perhaps the best path to characterizing rocky planets on Earth-like orbits.”
Perrin highlighted two intriguing results from the Gemini Planet Imager (GPI), an instrument designed not only to resolve the dim light of an exoplanet, but also analyze a planet’s atmospheric temperature and composition.
The first system observed with GPI was the well-known HR 8799 system, a large star orbited by four planets, located 130 light-years away. Previously, the Keck telescope had measured the atmosphere of one of the planets, HR 8799c, in six hours of observing time. But GPI matched that in only a half hour of telescope time and in less-than-ideal weather too. So the team quickly turned to the planet’s twin, HR 8799d.
“What we found really surprised us,” said Perrin. “These two planets have been known to have the same brightness and the same broadband colors. But looking at their spectra, they’re surprisingly different.”
Perrin and his colleagues think the likely culprit is clouds. It’s possible that one planet has a uniform cloud cover, whereas the other planet has a more patchy cloud cover, allowing astronomers to see deeper into the atmosphere. Perrin, however, cautions that this explanation is still under interpretation.
“The fact that GPI was able to extract new knowledge from these planets on the first commissioning run in such a short amount of time, and in conditions that it was not even designed to work, is a real testament to how revolutionary GPI will be to the field of exoplanets,” said GPI team member Patrick Ingraham from Stanford University in a news release.
Perrin’s presentation also introduced never-seen details in the dusty ring around the young star HR 4796A. GPI also has the unique ability of detecting only polarized light, which sheds light on different physical properties.
Although the details are fairly technical, “the short version is that reconciling the patterns we see in polarized intensity and in total intensity has forced us to think of this not as a very diffuse disk but one that is actually dense enough to partially opaque,” said Perrin.
The disk may be roughly analogous to one of Saturn’s rings.
“GPI now is moving into an exciting phase of full operations,” said Perrin, concluding his talk. “We’ll be opening up a lot of new discoveries hopefully over the next few years. And in the long run taking these technologies and scaling them to future 30-meter telescopes, and perhaps large telescopes in space, to continue direct imaging and push down toward the Earth-like planet regime.”
This isn’t a clone of our Solar System, but it’s close enough. Scientists eagerly scrutinized a young star system called HD 95086 to learn more about how dust belts and giant planets grow up together. This is an important finding for our own neighborhood, where the gas giants of Jupiter, Saturn, Uranus and Neptune are also wedged between dusty areas.
“By looking at other star systems like these, we can piece together how our own Solar System came to be,” stated lead author Kate Su, an associate astronomer at the University of Arizona, Tucson.
The system is about 295 light-years from Earth, and is suspected to have two dust belts: a warmer one (similar to our asteroid belt) and a cooler one (similar to the Kuiper Belt that has icy objects.) The system is host to at least one planet that is five times the mass of Jupiter, and other planets could also be hiding between the dusty lanes. This planet, called HD 95086 b, was imaged by the European Southern Observatory’s Very Large Observatory in 2013.
The next step was a comparison study with another star system called HR 8799, which also has two dusty rings and in this case, at least four planets in between. These planets have also been caught on camera. Comparing the structure of the two systems indicates that HD 95086 may have more planets lurking for astronomers to discover.
“By knowing where the debris is, plus the properties of the known planet in the system, we can get an idea of what other kinds of planets can be there,” stated Sarah Morrison, a co-author of the paper and a PhD student at the University of Arizona. “We know that we should be looking for multiple planets instead of a single giant planet.”
The researchers presented their work at the Division for Planetary Science Meeting of the American Astronomical Society in Tucson, Arizona. A press release did not disclose publication plans or if the work was peer-reviewed.
Late last year, astronomers using the Keck II telescope released the first direct image of a planetary system including four planets. These planets orbited the star HR 8799 and were taken in the J and L bandpasses which are in the near-infrared portion of the spectrum. Since then the team has collected new data using the same telescope, extending the spectral range into the mid-infrared portion of the spectra.
The new images are important to astronomers because this provides a more complete understanding of the distribution of radiation that the planets are emitting. This can be compared to models of planetary formation, allowing these young planets to act as a test bed. Previous comparison to models have suggested that these planets have cool, dusty atmospheres without the presence of methane or other common absorbing molecules.
The team hopes that the new observations will help distinguish between the various models that explain this deficiency of methane. Unfortunately, getting good observations in this portion of the spectra is challenging. In particular, at the Keck telescope, the design of the telescope itself makes observations especially challenging due to portions of the instrument themselves emitting in the infrared, masking the faint signals from the planet.
To bring out the planets, the team developed a new technique to help clean the images of the unwanted noise. They estimate that their new technique is nine times more efficient than previously used techniques. To do this, they moved the telescope slightly between images, allowing the patterns of interference to change between exposures, thereby making them more apparent and easier to remove.
When the results were analyzed and compared to models, the team found that they were in good agreement with predictions of planetary evolution for planets c and d. However, for planet b, the models predicted a planet with a radius that would be too small to account for the observed luminosity. The observations could be brought into agreement with the models by increasing the metallicity of the model.
With additional future observations, the team hopes to constrain these models and further investigate the atmospheres of these planets.
NOTE: I Emailed the authors of the paper to ask permission to reproduce the new image here, but have not gotten a reply. The one used above is the K and L band images from last year. To see the new ones, feel free to go to the paper directly.