Four points of light moving in partial concentric circles around a central disk are the protagonists of a video recently published on YouTube by astronomer Jason Wang, from Northwestern University, in the USA. And they are nothing less than planets orbiting a star 133.3 light years away from Earth.
The video, a time-lapse of 12 years of observations, is part of a long survey dedicated to understanding the planetary system of the star HR8799 – the first system seen directly by scientists, in 2008.
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Wang says he compiled these video observations not for any scientific reason, but just because he thought it was something incredible that should be shared with the general public.
“It is generally difficult to see planets in orbit,” said the scientist in an interview with the website Science Alert. “For example, it’s not apparent that Jupiter or Mars orbit our Sun because we live in the same system and don’t have a top-down view. Astronomical events happen too quickly or too slowly to capture on film. But, this video shows planets moving on a human timescale. I hope this allows people to enjoy something wonderful.”
Although the current count of confirmed exoplanets – i.e. planets outside the Solar System – is over 5,200, most of them have never been directly seen.
Astronomers find these worlds mostly through indirect methods, studying the effect they have on the host star. Regular, faint dips in the star’s light indicate an orbiting exoplanet passing between the observer and the star. Weak changes in the wavelength of the star’s light indicate the gravitational interaction between it and an exoplanet.
As Wang points out, the reason for this is that it is actually very difficult to see an alien planet directly. “They are very small and very faint compared to their host star; any light they emit or reflect is usually swallowed up by the star’s blazing light.”
Every once in a while, however, researchers get lucky. There are cases – like this one – where the exoplanets are large enough and far enough apart from their star, and the system oriented in such a way that if the star’s light is blocked or hidden (which is why HR8799 appears as a black disk) , we can see them as small bubbles of light.
Even rarer is seeing them executing their complex planetary pavement, simply because the timeframes of the orbits involved are much longer than the time since scientists first directly sighted the exoplanet.
Wang and his team, however, now have enough observational data from HR8799 for a time-lapse showing partial orbits, and this is what he compiled. “There’s nothing to be gained scientifically by watching systems in orbit in time-lapse video, but it helps others appreciate what we’re studying,” says Wang. “It can be difficult to explain the nuances of science in words. But showing science in action helps others understand its importance.”
The observations were collected using the WM Keck Observatory, and Wang applied adaptive optics to correct for the distorting effect of Earth’s atmosphere. The time-lapse was also processed to correct for time jumps between the data, showing the smooth orbital motion of the four exoplanets.
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What does the planetary system HR8799 look like
Wang explains that the black circle in the center is the 30-million-year-old young star, about 1.5 times the mass and 4.9 times the brightness of the Sun.
The innermost exoplanet is the gas giant HR8799e, with a mass 7.4 times that of Jupiter circling the star at a distance 16.25 times the separation between Earth and the Sun, for an orbital period of 45 years.
Moving outward, planet HR8799d has the mass of 9.1 Jupiters and orbits at 26.67 astronomical units (AU), for an orbital period of 100 years.
In turn, the exoplanet HR8799c is 7.8 Jupiters in mass, orbiting at a distance of 41.4 AU (just slightly wider than the separation between the Sun and Pluto) for an orbital period of 190 years. It has water in its atmosphere, scientists have discovered.
Finally, we have the HR8799b. 5.7 Jupiters in mass, distant from its star at 71.6 AU, with an orbital period of 460 years.
While the time lapse itself may not be scientifically revealing, Keck’s data collection certainly is.
A paper published in December last year found the possible existence of a fifth exoplanet, smaller and closer to the star than its siblings. The candidate is estimated to be about 4 to 7 times the mass of Jupiter, orbiting at a distance of between 4 and 5 AU, making direct detection difficult.
Wang and his colleagues are hard at work analyzing the system’s light. They hope to gain detailed information about the composition of not just the star, but the worlds around it. “In astrophysics, most of the time we are doing data analysis or testing hypotheses,” says Wang. “But this is the fun part of science. It inspires awe.”
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