Because of my fascination with exotic venues for astrobiology, I’ve always enjoyed Karl Schroeder’s novels. The Canadian writer explored brown dwarf planets as future venues for human settlement in Permanence (2002), and in his new book Lockstep (soon to be published by Tor, currently being serialized in Analog), Schroeder looks at ‘rogue’ planets, worlds that move through the galaxy without a central star. Imagine crimson worlds baked by cosmic radiation, their surfaces building up, over the aeons, the rust red complex organic molecules called tholins. Or consider gas giants long ago ejected from the system that gave them birth by close encounters with other worlds.
Objects like these and more are surely out there given what we know about gravitational interactions within planetary systems, and they’re probably out there in huge numbers. I’m not going to review how Lockstep uses them just yet — in any case, I haven’t finished the book — but we’ll return to its ingenious solution to time and distance problems in a future post. Right now I just want to mention that one of Schroeder’s characters muses upon ‘a hundred thousand nomad planets for every star in the galaxy.’ Now that’s some serious real estate.
If the number sounds like a novelistic exaggeration, it’s nonetheless drawn from recent work. Schroeder is invoking the work of Louis Strigari (Stanford University), who has studied the possibilities not only of planets ejected from their own systems but those that may form directly from a molecular cloud. The figure of 105 free-floating planetary objects for every main sequence star is from a 2012 paper in Monthly Notices of the Royal Astronomical Society (you can read more about Strigari’s ideas in ‘Island-Hopping’ to the Stars).
Rogue planets would be tricky to find but gravitational microlensing should help us set constraints on their actual numbers, and as we’ll see below, direct imaging has its uses. If rogue worlds are available in such quantities, we can imagine a starfaring culture capable of exploiting their resources. We can even speculate that a thick atmosphere that can trap infrared heat coupled with tectonic or radioactive heat sources from within could sustain elemental forms of life even in the absence of a star. Tens of thousands of objects in nearby interstellar space would obviously be a spur for exploration.
A Newly Found Orphan World
Eighty light years from Earth floats a solitary planet that has been discovered through its heat signature in data collected by the Pan-STARRS 1 wide-field survey telescope on Maui. In mass, color, and energy output, the world is similar to directly imaged planets. As you might expect, PSO J318.5-22, a gas giant about six times the mass of Jupiter, turned up during a search for brown dwarfs, delving into the datasets of a survey that has already produced about 4000 terabytes of information. The discovery was then followed up through multiple observations by equipment on nearby Mauna Kea, with spectra from the NASA Infrared Telescope Facility and the Gemini North Telescope indicating the young, low-mass object was not a brown dwarf.
Image: Multicolor image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. The planet is extremely cold and faint, about 100 billion times fainter in optical light than the planet Venus. Most of its energy is emitted at infrared wavelengths. The image is 125 arcseconds on a side. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium.
“We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” explained team leader Dr. Michael Liu of the Institute for Astronomy at the University of Hawaii at Manoa. “I had often wondered if such solitary objects exist, and now we know they do.”
The find is interesting on a number of levels, not least of which is that observations of gas giant planets around young stars have shown that their spectra differ from those of L- and T-class brown dwarfs. Young planets like these, according to the paper on this work, show redder colors in the near-infrared, fainter absolute magnitudes at the same wavelength and other spectral peculiarities that suggest the line of development between brown dwarfs and gas giant planets may not be as clear cut as once assumed. The paper makes clear how complex the issue is:
PSO J318.5−22 shares a strong physical similarity to the young dusty planets HR 8799bcd and 2MASS J1207−39b, as seen in its colors, absolute magnitudes, spectrum, luminosity, and mass. Most notably, it is the ﬁrst ﬁeld L dwarf with near-IR absolute magnitudes as faint as the HR 8799 and 2MASS J1207−39 planets, demonstrating that the very red, faint region of the near-IR color-magnitude diagram is not exclusive to young exoplanets. Its probable membership in the β Pic moving group makes it a new substellar benchmark at young ages and planetary masses.
A landmark indeed, and here the Beta Pictoris moving group, a collection of young stars formed about twelve million years ago, is worth noting. Beta Pictoris itself is known to have a young gas giant planet in orbit around it. The newly detected PSO J318.5−22 is lower still in mass than the Beta Pictoris planet and it is thought to have formed in a different way. The paper goes on:
We ﬁnd very red, low-gravity L dwarfs have ≈400 K cooler temperatures relative to ﬁeld objects of comparable spectral type, yet have similar luminosities. Comparing very red L dwarf spectra to each other and to directly imaged planets highlights the challenges of diagnosing physical properties from near-IR spectra.
The beauty of objects like these from an astronomical point of view is that we don’t have to worry about filtering out the overwhelming light of a parent star as we study them. Co-author Niall Deacon (Max Planck Institute for Astronomy) thinks PSO J318.5−22 will “provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth.” The discovery also gives us much to think about in terms of future explorations as we contemplate a cosmos in which perhaps vast numbers of planets move in solitary trajectories through the galaxy.
The paper is Liu et al., “The Extremely Red, Young L Dwarf PSO J318-22: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets,” in press at Astrophysical Journal Letters (preprint). Also intriguing is Abbot and Switzer, “The Steppenwolf: A proposal for a habitable planet in interstellar space” (preprint).