Brown dwarfs — like the one illustrated here – are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to Jupiter’s, with wind-driven, planet-size clouds. (NASA/JPL-Caltech)
Results from two very different papers in recent weeks have brought home one of the more challenging and intriguing aspects of large exoplanet hunting: that some exoplanets the mass of Jupiter and above share characteristics with small, cool stars. And as a result, telling the two apart can sometimes be a challenge.
This conclusion does not come from new discoveries per se and has been a subject of some debate for a while. But that borderland is becoming ever more tangled as discoveries show it to be ever more populated.
The first paper in The Astrophysical Journal described the first large and long-lasting “spot” on a star, a small and relatively cool star (or perhaps “failed star”) called an L dwarf. The feature was similar enough in size and apparent type that it was presented as a Jupiter-like giant red spot. Our solar system’s red spot is pretty well understood and the one on star W1906+40 certainly is not. But the parallels are nonetheless thought-provoking.
“To my mind, there are important similarities between what we found and the red spot on Jupiter,” said astronomer John Gizis of the University of Delaware, Newark. “Both are fundamentally the result of clouds, of winds and temperature changes that create huge dust clouds. The Jupiter storm has been going for four hundred years and this one, well we know with Hubble and Spitzer that it been there for two years, but it’s probably more.”
A far cry from 400 years, but the other similar storms and spots identified have been on brown dwarfs — failed stars that start hot and burn out over a relatively short time. Gizis said some large storms have been detected on them but that they’re gone in a few days.
The dust and wind storm on the L dwarf W1906+40 rotates around the cool star every nine hours and is large enough to hold three Earths. L-dwarfs mark the boundary between real stars and “failed stars.” . Most known L dwarfs are brown dwarfs, also known as “failed stars” because they never sustain atomic fusion, but the most massive L dwarfs can fuse hydrogen atoms and generate energy like our sun. (JPL/NASA-Caltech)
The second article came from Alexandre Santerne of the Instituto de Astrofísica e Ciências do Espaço, Portugal and Aix Marseille University, France, and was shared and widely discussed at the recent Extreme Solar Systems meeting in Hawaii.
In the Astronomy & Astrophysics paper, the researchers report that a high percentage (55 percent) of the very large exoplanet “candidates” listed by the Kepler mission are in fact not exoplanets. Santerne and colleagues spent a year’s worth of nights between 2010 and 2015 observing, via the radial velocity method, 129 of Kepler’s more than 4,000 planet candidates. Their tool was the SOPHIE spectrograph at Haute-Provence Observatory in southeastern France.
The Kepler science team has long predicted that the “false positive” rate for these very large radii planets would be high — a projected 30-40 percent rate for candidates larger than Jupiter versus less than 10 percent false positive rate for candidates smaller than Jupiter. But this even higher percentage came initially as something of a worrisome surprise.
Many of what the Santerne team described as “false positives” were determined to be multi-star systems (rather than a star with planets) while three were identified as brown dwarfs, those small, cool failed suns.
Said team member Vardan Adibekyan of the Centre for Astrophysics of the University of Porto: “Detecting and characterizing planets is usually a very subtle and difficult task. In this work, we showed that even big, easy to detect planets are also difficult to deal with.”
While finding many false positives, the Santerne team also confirmed 45 Kepler very large planet candidates, fifteen more than had been confirmed before.
Size comparison of celestial objects from our sun to Earth. (NASA/JPL-Caltech/UCB)
Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission, said that at first glance the reported false positive rates seemed higher than expected based on predictions by the Kepler team, in particular the modeling work of astrophysicist Timothy Morton of Princeton.
But after a careful read and some number crunching, Batalha said she came away confident that the new results do not reflect any flaws in the planet identification process itself and, in fact, agree with predictions. The apparent rise in the false positive rate, she said, can be attributed to a more liberal inclusion of larger exoplanet “candidates” initiated in 2014 by the Kepler mission.
Previously, planet candidates more than twice the radius of Jupiter were all discarded because no planets above that line had ever been detected — they were deemed “astrophysical false positives”. But they were returned to the “candidate” list a year ago so that scientists could explore the transition between giant planets and brown dwarfs and small stars. Once these larger-than-two-Jupiter “candidate” planets were folded back into the Jovian planet group, Batalha says, the false positive rate for the group naturally shot up. Which is predictable, since no two-Jupiter planets were identified by the Santerne group.
Nonetheless, she said, the results reflect and illustrate the complex nature of large exoplanet detection and characterization. “The truth is that we don’t know a lot about the transition from giant planets to stars. It’s an important subject and this team is one of the few working on it.”
Colors plotted here represent the average expected false positive rate for candidates of a given size (radius on the y-axis) and orbital period (x-axis). For candidates smaller than Jupiter, the expected false positive rate is less than 10%. For planets between one and two Jupiter radii, the false positive rate jumps up to 38%. For objects larger than twice the size of Jupiter, the false positive rate increases to 90%. White points show the properties of the candidates observed by the Santerne team. (NASA Exoplanet Archive/N. Batalha, T. Morton
As determined by the International Astronomical Union, any celestial object with a mass greater than 13 Jupiters should be considered a star.But according to Jonathan Fortney, an exoplanet and brown dwarf theorist at the University of California, Santa Cruz, this definition leaves a lot of researchers cold because it doesn’t take into account how the object was formed.
Did it form in a giant molecular cloud (like most stars)? Or in orbit around a parent star, by slowly adding on large amounts of gas, atop a solid core of rock and ice (like most planets)? Or as a result of gravitational instability in a disk (a theory that suggests the formation of massive gas giant planets as the result of a quick pulling together of disk material to form dense clumps)?
“It seems clear that star formation can make objects less massive than ten Jupiters and we can see planets more massive than several Jupiters in disks around stars. So there’s an overlap here, and we don’t always know when star formation stops and planet formation starts,” Fortney said. “That why it’s so important to learn about the composition and evolution of the objects to figure out what they are.”
Artist’s conception of the clouds on Kepler-7b, compared for size with Jupiter (right). Many exoplanets and brown dwarfs have mostly hydrogen-helium atmospheres that are covered in layers of mineral dust, while Jupiter’s hydrogen-helium atmosphere has clouds of ammonia. (NASA/JPL-Caltech/MIT)
And of particular interest in that borderland are brown dwarfs, convincingly identified only twenty years ago.
As Fortney explained, brown dwarfs are formed in the same vast clouds that produce stars by the hundreds, but don’t have sufficient mass to build the internal pressure needed to begin the nuclear fusion of hydrogen that defines a star. Still, the gravitational energy of a brown dwarf does get converted into heat and so they can warm their surroundings before cooling like embers leaving a fire. Some researchers even hold that planets could form around brown dwarf and protoplanetary disks have already been found around a few of them.
What particularly fascinates Fortney about brown dwarfs is that they have atmospheres and winds and weather, and as a result offer some potential insights into larger exoplanets, especially those surrounded by thick dust clouds.
This overlay of suspended minerals (sometimes exotic metals like aluminum oxide and magnesium-rich forsterite — a form of silicate rock — and irons) have made it very difficult if not impossible to look spectroscopically at the atmospheres of many exoplanet. But depending on the temperatures and compositions of the dust clouds, astronomers sometimes have more luck looking through the clouds and haze of brown dwarfs.
But still, the process of getting information about distant atmospheres is painstaking and Fortney said his work with brown dwarfs provides “a window into just difficult it is and will be” with exoplanets. Basic questions like temperatures, what kinds of molecules are present and in what abundances — they’re all veiled by the dust clouds.
(Find a panel discussion about the brown dwarf-exoplanet connection here.)
Artist’s rendering of a brown dwarf surrounded by cosmic dust and gases. ALMA (ESO/NAOJ/NRAO)/M. Kornmesser
Progress, however, is being made, both in terms of technical approaches to “seeing” through the clouds, and the science of these objects. Even gigantic exoplanets appear to have clouds and dynamic atmospheres, Fortney said, “and I think we’ll see that across the board.”
Batalha also identified a related bit of progress. The Santerne paper identified three brown dwarfs in the Kepler candidate list, she wrote, and so they produced the beginning of an occurrence rate for brown dwarfs. In addition, the paper published an occurrence rate for warm Jupiter-size planets within one astronomical unit or AU (roughly the distance from the sun to Earth) of their own sun.
Putting the two observations together, and you reach the conclusion that warm Jupiters are 15 times more common than brown dwarfs in similar one AU orbits.
That, she said, is the intriguing news coming from the giant planet/failed star borderland.
