March 28, 2023

Watch four exoplanets dance around their star

Four points of light moving in a few concentric circles around a central disk are the stars of a video recently posted on YouTube by astronomer Jason Wang, from Northwestern University, in the US. 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.


Wang says he compiled these video observations not for any scientific reason, but simply because he thought it was something incredible that should be shared with the general public.

“It’s generally difficult to see orbiting planets,” the scientist said in an interview with the website Science Alert. “For example, it is not obvious that Jupiter or Mars orbit our Sun because we live in the same system and do not have a top-down view. Astronomical events happen too quickly or too slowly to be captured on film. But this video shows planets moving on a human time scale. I hope this allows people to enjoy something wonderful.”

Although the current number of confirmed exoplanets – that is, planets outside the Solar System – is over 5,200, most of them have never been seen directly.

Astronomers find these worlds mostly through indirect methods, studying the effect they have on the host star. Regular, faint dips in the starlight indicate an orbiting exoplanet passing between the observer and the star. Faint 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 bright starlight.”

Every once in a while, though, researchers get lucky. There are cases – like this one – where exoplanets are large enough and far enough from their star, and the system is oriented in such a way that if the star’s light is blocked or obscured (which is why HR8799 appears as a black disk), we can we see them as little bubbles of light.

It’s even rarer to see them execute their complex planetary trajectories, simply because the time frames of the orbits involved are so much longer than the time since scientists first saw the exoplanet directly.

Wang and his team, however, now have enough observational data from HR8799 for a time-lapse showing some orbits, and that’s what he put together. “You can’t gain anything scientifically by watching orbiting systems 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 the Earth’s atmosphere. The time-lapse was also processed to correct the time jumps between the data, showing the smooth orbital motion of the four exoplanets.

Read more:

What the HR8799 planetary system looks 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 luminosity of the Sun.

The innermost exoplanet is the gas giant HR8799e, with a mass 7.4 times that of Jupiter orbiting the star at a distance 16.25 times the distance between Earth and the Sun, with an orbital period of 45 years.

Moving outward, planet HR8799d has a mass of 9.1 Jupiters and orbits at 26.67 astronomical units (AU), with an orbital period of 100 years.

In turn, exoplanet HR8799c has a mass of 7.8 Jupiters, orbiting at a distance of 41.4 AU (slightly wider than the separation between the Sun and Pluto) with an orbital period of 190 years. It has water in its atmosphere, scientists have discovered.

Finally, we have the HR8799b. 5.7 Jupiter in mass, at a distance from its star of 71.6 AU, with an orbital period of 460 years.

While the time lapse itself may not be scientifically revelatory, Keck’s data collection certainly is.

A paper published last December 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 from a distance of between 4 and 5 AU, making direct detection difficult.

Wang and his colleagues are working hard to analyze the system’s light. They hope to gain detailed information about the composition of not only the star, but also the worlds around it. “In astrophysics, most of the time we’re doing data analysis or testing hypotheses,” says Wang. “But that’s the fun part of science. It’s awe-inspiring.”

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