‘Planemos’ are planetary mass objects not much larger or heavier than Jupiter. The emerging technical term for them is ‘isolated planetary mass objects’ (IPMO), although the nomenclature is still evolving. Back in 2006, Ray Jayawardhana (University of Toronto) challenged the American Astronomical Society’s Calgary meeting to consider how our definition of ‘planet’ is blurred by planemos that act much like little solar systems. Consider Jupiter itself, a small system doubtless born with its own disk of dust and gas that produced the raw materials for its larger moons.
Backing up such thinking was the brown dwarf 2M1207, known to have a planetary companion eight times the mass of Jupiter and now shown to be surrounded by a disk of its own. Thus it comes as no surprise that Jayawardhana, following up this work with Alexander Scholz (University of St Andrews), has been using the Spitzer Space Telescope to study eighteen planemos in a star cluster in Orion. At three million years old, young stars tend to be surrounded by gas and dust that glows in the infrared, a marker of the raw materials for planetary formation. About one-third of the planemos under study show similar disks.
So ‘planetary’ systems may form even in the presence of a planemo instead of a central star. What’s left hanging is the question of where the planemos come from in the first place. They’re smaller than brown dwarfs (with masses close to or below the deuterium-burning limit) and may well be planets expelled from young planetary systems. Or perhaps, say the scientists, they’re stellar embryos ejected from mini-clusters or multiple star systems. Whatever the case, the σ Orionis cluster offers the largest population of these objects yet identified.
From the paper (internal references deleted for brevity):
“…our results fit into previous claims for a T Taurilike phase in the planetary mass regime… Disk fractions and thus lifetimes are similar for objects spanning more [than] two orders of magnitude in mass…possibly indicating that stars, brown dwarfs, and IPMOs share a common origin. Star formation theory thus has to account for a number of objects with masses below the Deuterium burning limit.”
Interesting work, because in recent years the study of brown dwarfs has shown that they have what Jayawardhana and Scholz call ‘circum-sub-stellar disks’ with life times between five and ten million years, which makes them not dissimilar to other kinds of stars. We’re deep into what’s known as the initial mass function (IMF), which describes the distribution of stellar masses in a formation event in a specified volume of space. Just how that IMF is extended to encompass findings like these, moving down into the realm of the giant planets, will help us to probe its apparent universality in relation to star formation theory.
Thus the similarities between objects of vastly different sizes has to be factored into our thinking about how stars form. Again we note, in the presence of those gas and dust disks, the apparently ubiquitous phenomenon of planetary formation. Are we really looking at a universe that seems to seed almost every kind of star and even giant planets with companions? The paper, to be published in The Astrophysical Journal Letters, is “Dusty disks at the bottom of the IMF,” available online.