The Gemini team’s research strengthens the model that massive planets are born as the result of the slow accumulation of material swirling around a baby star. In contrast, their study also indicates that a brown dwarf is born as the result of a rapid gravitational collapse of their natal cloud. “It’s a bit like the difference between a gentle light rain and a thunderstorm,” commented Dr. Macintosh in the June 11, 2019 Gemini Press Release.
“With six detected planets and three detected brown dwarfs from our survey, along with unprecedented sensitivity to planets a few times the mass of Jupiter at orbital distances well beyond Jupiter’s, we can now answer some key questions, especially about where and how these objects form,” Dr. Nielson explained in the same Press Release.
Hence, this new research may solve the longstanding mystery as to whether brown dwarfs (intermediate-mass objects) are born like stars or planets.
“What the GPIES team’s analysis shows is that the properties of brown dwarfs and giant planets run completely counter to each other. Whereas more massive brown dwarfs outnumber less massive brown dwarfs, for giant planets the trend is reversed: less massive planets outnumber more massive ones. Moreover, brown dwarfs tend to be found far from their host stars, while giant planets concentrate closer in. These opposing trends point to brown dwarfs forming top- down, and giant planets forming bottom-up,” explained study co-author Dr. Eugene Chiang in the June 11, 2019 Gemini Press Release. Dr. Chiang is a professor of astronomy at the University of California Berkeley.
Of the 300 stars studied so far, 123 are at least 1.5 times solar-mass. One of the most important results to come out of the GPI survey is that all the stars that host detected planets are among these higher-mass stars–even though it is easier to observe a giant planet orbiting a dimmer, more Sun-like star. Astronomers have long suspected this relationship, but the GPIES survey has unambiguously confirmed it. This new finding also adds more credibility to the bottom-up formation model for planets.
One of the study’s biggest surprises has been how different other planetary systems are from our own. Our Solar System is neatly composed of a quartet of small rocky planets inhabiting the inner region, and a quartet of giant gaseous planets inhabiting the colder outer domain. However, the very first exoplanets to be discovered reversed our Solar System’s model. The first exoplanets to be discovered a generation ago were giant planets situated much closer to their parent-stars than moon-sized Mercury–our innermost planet–is to our Sun. In addition, radial velocity studies–which are based on the observation that a star experiences a gravitationally induced “wobble” when it is circled by a planet–indicated that the number of giant planets increases with distance from the parent-star out to approximately Jupiter’s orbit. In contrast to the radial velocity observations, the GPIES team’s early results–which takes into account larger distances–has demonstrated that giant planets actually become less numerous farther out.
“The region in the middle could be where you’re most likely to find planets larger than Jupiter around other stars, which is very interesting since this is where we see Jupiter and Saturn in our own Solar System,” Dr. Nielsen explained in the June 11, 2019 Gemini Press Release. Indeed, the location of Jupiter in our Solar System may fit the overall exoplanet trend.
In our Sun’s own planetary family, Jupiter and Saturn are the two gas-giants of the outer Solar System, with Saturn being the smaller of the enormous gas-blanketed duo. The other two giants in this distant cold domain are Uranus and Neptune, which are both categorized as ice-giants rather than gas-giants. This is because the frigid duo have larger solid cores than Jupiter and Saturn (which may be all gas) and thinner gaseous atmospheres.
An additional surprise, derived from all exoplanet surveys, is how rare giant planets seem to be around Sun-like stars, and how very different other Solar Systems are. NASA’s exoplanet-hunting Kepler mission–which searched for Earth-like planets beyond our own Sun–found many more close-in small planets–two or more “super-Earth” planets per Sun-like star, densely packed into inner solar systems much more crowded than our own. This finding suggests that GPI would discover a dozen giant planets–or more. However, GPI only detected six. Putting all of this new information together, giant planets may circle only a minority of stars like our own Sun.
In January 2019, GPIES observed its 531st–and last–new star, and the team is currently following up the remaining candidates in order to determine which are really planets, and which are distant background stars that merely appear to be giant planets.
Upcoming telescopes, such as NASA’s James Webb Space Telescope,WFIRST, the Giant Magellan Telescope , the Thirty Meter Telescope, and the Extremely Large Telescope, should have the ability to push the boundaries of study. That is because these next-generation telescopes can image planets much closer to their parent-stars, and will also overlap with other methods, thus producing a full accounting of giant planet and brown dwarf populations from 1 to 1,000 astronomical units (AU). One AU is equivalent to the average Earth-Sun distance which is 93,000,000 miles.