Giant Gas Planet or Brown Dwarf – Where Does One Draw the Line?
The range of sizes of a brown dwarf compared to Jupiter and the Sun and the Earth (to scale). Brown Dwarfs are more massive than planets but less massive than stars. But they have similar diameters to planets such as Jupiter. Brown dwarfs are a class of objects halfway between giant planets, like Jupiter, and stars. All objects with a mass greater than about 75 times that of Jupiter (7% of the Sun’s mass) are stars, and mainly burn hydrogen for their entire lifetimes. During this phase of nuclear combustion, their luminosity remains quite stable. (Image Credit: NASA, JPL-Caltech, UCB)
Giant Gas Planet or Brown Dwarf – Where Does One Draw the Line?
In our Solar System, Jupiter is the largest planet, being about 318 times as massive as the Earth and lying about five times farther from the Sun than does the Earth. Brown dwarfs are similar in many ways to Jupiter-like gas giants, but range from 13 to 90 times the mass of Jupiter… And while they can be up to a tenth the mass of the Sun, they lack the nuclear fusion in their core to burn as a star, so they lie somewhere between a diminutive star and a super-planet. Based on preliminary results from a new Gemini Observatory survey of 531 stars with the Gemini Planet Imager (GPI), it appears more and more likely that large planets and brown dwarfs have very different roots. While massive planets form due to the slow accumulation of material surrounding a young star, brown dwarfs come about due to rapid gravitational collapse.
The GPI Exoplanet Survey (GPIES), one of the largest and most sensitive direct imaging exoplanet surveys to date, is still ongoing at the Gemini South telescope in Chile. “From our analysis of the first 300 stars observed, we are already seeing strong trends,” said Eric L. Nielsen of Stanford University, who is the lead author of the study.
In November 2014, GPI Principal Investigator Bruce Macintosh, of Stanford University, and his international team set out to observe almost 600 young nearby stars with the newly commissioned instrument. GPI was funded with support from the Gemini Observatory partnership, with the largest portion from the US National Science Foundation (NSF). The NSF and the Canadian National Research Council (NRC - also a Gemini partner) funded researchers participating in GPIES.
Imaging a planet around another star is a difficult technical challenge possible with only a few instruments. Exoplanets are small, faint, and very close to their host star. Distinguishing an orbiting planet from its star is like resolving the width of a dime from several miles away. Even the brightest planets are ten thousand times fainter than their parent star. GPI can see planets up to a million times fainter, much more sensitive than previous planet-imaging instruments. “GPI is a great tool for studying planets, and the Gemini Observatory gave us time to do a careful, systematic survey,” said Macintosh.
Most exoplanets discovered thus far, including those found by NASA’s Kepler spacecraft, have been found via indirect methods, such as observing a dimming in the star’s light as the orbiting planet eclipses its parent star, or by observing the star’s wobble as the planet’s gravity tugs on the star. These methods have been very successful, but they only probe the central regions of planetary systems. Those regions outside the orbit of Jupiter, where the giant planets are in our Solar System, are usually out of their reach. GPI, however, endeavors to directly detect planets in this parameter space by taking a picture of them alongside their parent stars.
The Gemini results support those from these other techniques, including a recent study of exoplanets discovered by the radial velocity method that found the most likely separation for a giant planet around Sun-like stars is about 3 AU. The finding that brown dwarfs occur with a frequency of only about 1%, independent of stellar mass, is also consistent with previous results from direct imaging surveys.
GPIES is now coming to an end. From the first 300 stars, GPIES has detected six giant planets and three brown dwarfs. “This analysis of the first 300 stars observed by GPIES represents the largest, most sensitive direct imaging survey for giant planets published to date,” added Macintosh.
Brown dwarfs are more massive than planets, but not massive enough to fuse hydrogen like stars. “Our analysis of this Gemini survey suggests that wide-separation giant planets may have formed differently from their brown dwarf cousins,” Nielsen said.
The team’s paper advances the idea that massive planets form due to the slow accumulation of material surrounding a young star, while brown dwarfs come about due to rapid gravitational collapse. “It’s a bit like the difference between a gentle light rain and a thunderstorm,” said Macintosh.
“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,” Nielsen said.
This discovery may answer a longstanding question as to whether brown dwarfs - intermediate-mass objects - are born more like stars or planets. Stars form from the top down by the gravitational collapse of large primordial clouds of gas and dust, while planets are thought, but have not been confirmed, to form from the bottom up by the assembly of small rocky bodies that then grow into larger ones, a process also termed “core accretion.”
“What the GPIES team’s analysis shows is that the properties of brown dwarfs and giant planets run completely counter to each other,” said Eugene Chiang, professor of astronomy at the University of California Berkeley and a co-author of the paper. “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.”
Of the 300 stars surveyed thus far, 123 are at least 1.5 times more massive than our Sun. One of the most striking results of the GPI survey is that all hosts of detected planets are among these higher-mass stars, even though it is easier to see a giant planet orbiting a fainter, more Sun-like star. Astronomers have suspected this relationship for years, but the GPIES survey has unambiguously confirmed it. This finding also supports the bottom-up formation scenario for planets.
One of the study’s greatest surprises has been how different other planetary systems are from our own. Our Solar System has small rocky planets in the inner parts and giant gas planets in the outer parts. But the very first exoplanets discovered reversed this trend, with giant planets skimming closer to their stars than does moon-sized Mercury. Furthermore, radial-velocity studies, which rely on the fact that a star experiences a gravitationally induced “wobble” when it is orbited by a planet, have shown that the number of giant planets increases with distance from the star out to about Jupiter’s orbit. But the GPIES team’s preliminary results, which probe still larger distances, have shown that giant planets 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," added Nielsen, “which is very interesting since this is where we see Jupiter and Saturn in our own Solar System.” In this regard, the location of Jupiter in our own Solar System may fit the overall exoplanet trend.
But a surprise from all exoplanet surveys is how intrinsically rare giant planets seem to be around Sun-like stars, and how different other solar systems are. The Kepler mission discovered far more small and close-in planets -- two or more “super-Earth” planets per Sun-like star, densely packed into inner solar systems much more crowded than our own. Extrapolation of simple models suggested GPI would find a dozen giant planets or more, but it only saw six. Putting it all together, giant planets may be present around only a minority of stars like our own.
In January 2019, GPIES observed its 531st, and final, new star, and the team is currently following up the remaining candidates to determine which ones are planets and which ones are distant background stars impersonating giant planets.
The next-generation telescopes (such as NASA’s James Webb Space Telescope and WFIRST mission, the Giant Magellan Telescope, Thirty Meter Telescope, and Extremely Large Telescope) should be able to push the boundaries of study, imaging planets much closer to their star and overlapping with other techniques, producing a full accounting of giant planet and brown dwarf populations from 1 to 1000 AU.
“Further observations of additional higher mass stars can test whether this trend is real,” said Macintosh, “especially as our survey is limited by the number of bright, young nearby stars available for study by direct imagers like GPI.”
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