The Formative Years
Giant Planets vs. Brown Dwarfs
11 June 2019
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.
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, published in The Astronomical Journal.
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.
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, has 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 are truly planets and which 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 1,000 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.”
GPI is specifically designed to search for planets and brown dwarfs around other stars, using a mask known as a coronagraph to partially block a star’s light. Together with adaptive optics correcting for turbulence in the Earth’s atmosphere and advanced image processing, researchers can search the star’s neighborhood for Jupiter-like exoplanets and brown dwarfs up to a million times fainter than the host star.
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 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.
An early success of GPIES was the discovery of 51 Eridani b in December 2014, a planet about two-and-a-half times more massive than Jupiter, that orbits its star beyond the distance that Saturn orbits our own Sun. The host star, 51 Eridani, is just 97 light-years away, and is only 26 million years old (nearby and young, by astronomy standards). The star had been observed by multiple planet-imaging surveys with a variety of telescopes and instruments, but its planet was not detected until GPI’s superior instrumentation was able to suppress the starlight enough for the planet to be visible.
GPIES also discovered the brown dwarf HR 2562 B, which is at a separation similar to that between the Sun and Uranus, and is 30 times more massive than Jupiter.
Most exoplanets discovered thus far, including those found by NASA’s Kepler spacecraft, are 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.
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