The halfway analysis of the planetary survey of the Gemini Planet Imager suggests that our solar system could have rare qualities that could be linked to the habitability of the Earth.
By Taylor Kubota
In the last four years, an instrument connected to a telescope in the Chilean Andes – known as Gemini Planet Imager – has set its sights on 531 stars in search of new planets. The team, led by Stanford University, is now publishing the initial results of the first half of the survey, published June 12 in The Astronomical Journal.
The survey photographed six planets and three brown dwarfs in orbit around these 300 stars and offered new details about the planets of Jupiter, which could influence theories about how the Earth was formed and became habitable.
"In the last twenty years, astronomers have discovered all these solar systems that are very different from ours," he said Bruce Macintosh, professor of physics at Stanford in School of Letters and Sciences. "The question we want to understand in the end is: are there Earth-like planets, similar to Earth, out there? And one way to answer is to understand how other solar systems are formed."
Unlike other planet hunting techniques, which are based on the search for signs of a planet – such as the effect of its gravity on the parent star – rather than on the planet itself, the Gemini Planet Imager takes direct images, collecting the faint planet with the glow of a star a million times brighter.
"The giant planets in our solar system live between five and 30 times the orbital distance of the Earth, and for the first time we are probing a similar region around other stars," he said Eric Nielsen, a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology and lead author of the article. "It's really exciting to be able to start putting together a census of Jupiter's larger planets in the outer solar systems of some of our nearby stars."
Perhaps a special system
Most other techniques probe the inner parts of solar systems. But the Gemini Planet Imager focuses in particular on the exoplanets that are large, young and far from the star that orbit. In our solar system, the external parts are the home of the giant planets. The Gemini Planet Imager helps researchers better understand if other solar systems have planets like Jupiter. However, while the Gemini Planet Imager is one of the most sensitive planet reproducers, there are still objects that escape it and the planets that this team currently sees are more than twice the mass of Jupiter.
In the first half of the survey, the Gemini Planet Imager found fewer exoplanets than expected researchers. However, the exoplanets they saw contributed to one of their strongest results: each of the six planets orbited a large, bright star – despite the fact that the planets are easier to see near weak stars. This definitely shows that giant planets in wide orbit are more common around high-mass stars, at least 1.5 times more massive than the sun. Meanwhile, for sun-like stars, Jupiter's larger cousins are much rarer than the small planets discovered near their star by missions like the NASA Kepler.
"Given what we've seen so far and other surveys, our solar system doesn't look like other solar systems," Macintosh said. "We don't have as many planets crammed so close to the sun as they do to their stars and now we have proven evidence that another way we might be rare is to have this kind of planets of Jupiter and above."
Although exact exoplanets equivalent to Jupiter are just beyond the reach of their instruments, not finding even a hint of something similar to Jupiter around these 300 stars leaves open the possibility that our Jupiter is special.
Another result of the first half of the survey is that brown dwarfs – larger objects than planets but smaller than stars – are a very distinct population from the planets. This could indicate a different formation mechanism for this class of objects, suggesting that brown dwarfs are more like failed stars than super-dimensional planets.
Combined with other techniques, this card identifies a distance from a star to which the number of giant planets passes from increasing to decreasing – to about 5-10 astronomical units (an astronomical unit is the distance from the sun to the Earth).
"The region in the middle could be where it is more likely to find larger planets than Jupiter around other stars," Nielsen added, "which is very interesting given that this is where we see Jupiter and Saturn in our solar system."
All three main findings support the hypothesis that giant planets probably form "from bottom to top" by accumulating particles around a solid nucleus, while brown dwarfs probably form "top down" as a result of huge gravitational instabilities in the disk of gas and dust from which a solar system develops.
Working towards the Earth
The Gemini Planet Imager Exoplanet Survey (GPIES) observed its 531st and last star in January 2019. The Gemini Planet Imager team is now working to make the instrument more sensitive to the smaller and fresher exoplanets that orbit closer to their own . Meanwhile, investigations that can indirectly observe those exoplanets are shifting their sensitivity to the outside. In the not too distant future, the two should gather in the corners of the space where a solar system like ours could still hide. Whichever instrument is the first to be able to directly see a world similar to the Earth, Macintosh imagines that it will be, at least in part, a descendant of the Gemini Planet Imager.
"Right now, we see these planets as blurry red blobs. One day, it will be a blurry blue blob. And that small, blurry, blue blob will become an Earth," said Macintosh. "Reaching the Earth will require a space mission that is probably about 20 years away. But when it flies, it will use a spectrograph like the one we built and deformable mirrors like the one we have and software with lines of code we wrote."
More immediately, the members of the GPIES team plan to publish further results on the survey, including information they have gathered about the atmospheres of the exoplanets they have seen, and end up analyzing the data obtained during the second half of the survey.
"I helped take the first research images of the GPIES planet four and a half years ago," he said Robert De Rosa, researcher at the Kavli Institute of Astrophysics and Particle Cosmology and co-author of the newspaper, who spent many nights observing with the Gemini Planet Imager in Chile and remotely from Stanford. "It is bittersweet to see it coming to an end".
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