July 15, 2005 | David F. Coppedge

Planet Orbiting Triple Star Tightens Noose on Planet Formation Theories

The discovery of a planet orbiting a triple star system (see JPL Press Release), described by Maciej Konacki in Nature,1 has delivered a severe challenge to theorists.  In short, the environment is “particularly prohibitive” for planet formation.  This Jupiter-size planet should not be there.
    Planet-formation theories have taken a triple whammy lately.  The discovery in recent years of so-called “hot Jupiters” (giant planets close to their parent stars – see 05/07/2004) was unexpected; it caused a major reconsideration about where and how gas giants form.  Prior to the indirect observation of planets like 51 Pegasi, which is closer to its star than Mercury to our sun, it was thought impossible that a Jupiter-class planet could form in a tight orbit, because the gases like hydrogen and helium that make up the bulk of such planets could only be retained beyond the “snow line” of about 3 AU.2  This led to a radical reinterpretation of the core-accretion hypothesis: planets formed far out, then migrated inward (see 05/16/2003 entry).
    The second whammy was the revival of the disk-instability hypothesis as a strong competitor to the core-accretion hypothesis, with proponents of each arguing not for the strengths of their own views, but against the weaknesses of their opponents’ views (see 09/22/2003, and Quick Takes following the 07/25/2003 entry).  Added to these headaches have been ongoing discoveries of planets where they shouldn’t be, like around a binary star (08/24/2004), around a white dwarf in a globular cluster (07/10/2003), in wildly elliptical orbits (07/21/2003), orbiting young stars (11/11/2004 and 05/28/2004) and even wandering alone (11/29/2003).  In addition, there seems to be no correlation between dust disks and planets (10/18/2004), and many stellar environments seem downright hostile to planets (07/06/2004 and 04/26/2001).  This scattering of strange observations led Stuart Ross Taylor last year to lament the lack of order in planetary science and to call the origin of the solar system “one of the oldest unsolved problems in science” (07/29/2004).
    This third whammy appears to be a crushing blow.  Planet-formation theories began optimistically with the nebular hypothesis of Pierre Laplace in the 18th century, but each new observation seems to raise the stakes.  Last August (08/27/2004), the planet found around a binary was tentatively rationalized because the two host stars were widely separated (56 year orbital period), leaving enough space for a dust disk to supply planet-building material around one star that would not be perturbed by the other.  The binary period of this new pair named HD 188753 is less than half that.  German astronomers Hatzes and Wuchteri, commenting on this discovery in the same issue of Nature,3 explain the difficulty:

The binary orbital period of HD 188753 is just 25.7 years, and the orbital separation of the stars, both of Sun size, is a mere 12.3 AU – about the distance from the Sun to Saturn.  Konacki’s velocity measurements reveal that the primary star (the more massive star, denoted HD 188753A) has a planetary companion of a minimum of 1.14 Jupiter masses that orbits the star every 3.35 days at a distance of about 0.05 AU.  Yet according to the orbital migration theory, this planet should not exist.  The secondary star is so close that its gravitational pull would have stripped away the protoplanetary disk of the primary star – where, even if it later migrated, the planet must have formed – reducing the disk to a radius of just 1.3 AU.  But within this radius, ices are unlikely to last and so cannot contribute to the formation of a massive core.  The alternative explanation – that the planet formed where it is – would challenge the standard picture, but runs into the problem of where the necessary solid material came from. (Emphasis added in all quotes.)

Konacki, the discoverer, mentions but then quickly dismisses simplistic explanations:

It appears that within the favoured scenario for the formation of hot Jupiters, the planet around HD 188753A could not be formed (unless for some reason the orbit of HD 188753 AB was very different during a planet formation phase).  One possible explanation is that the snowline can indeed be as close as ~1 au from the star.  Another possibility is that hot Jupiters form in situ, near their current orbital locations.  However, the problem is probably more challenging.  It has been suggested that planet formation in binary systems may be less efficient because the stirring induced by the secondary can significantly heat up the protoplanetary disk.  This may hold true for both the standard core accretion scenario and the recently revived gravitational collapse scenario [i.e., disk-instability model] of giant-planet formation.  Presumably, the environment of HD 188753 is particularly prohibitive given a small semi-major axis and the high mass of the secondary.

He just leaves it at that.  Hatzes and Wuchteri admitted in their commentary that the discovery of hot Jupiters “shook long-held conceptions of planetary-system formation,”  and that the new discovery “places severe constraints” on such theories.  What do they suggest?  A popular word, in our culture – diversity:

Although the planet in the HD 188753 system presents a conundrum to theorists, there might be an easy way out: abandon the make-do-and-mend migration theory to Occam’s razor, and accept that not all planet-forming nebulae are similar to the solar nebula.  Large and small protoplanetary nebulae of the same mass might differ only in their total angular momentum, such that in smaller nebulae more mass is closer in – nursing young giants.
    The giant planets that orbit other stars exist in a diversity of systems and most are unlike the system of planets found around our own Sun.  In our view, the diversity of planetary systems probably reflects a diversity of protoplanetary nebulae, and wherever sufficient mass is available, planets, even giant ones, may form.  The neglected majority of double stars could thus fill the Galaxy with planets.

The problem with their suggestion is that it leaves theorists at square one, with no single theory that could ever hope to explain the formation of such diversity of planets, and with nothing better than an armchair speculation that material will coalesce somehow in spite of the problems with temperature and turbulence.  The NASA press release apparently took “the easy way out” and spun the story optimistically, quoting Dr. Shri Kulkarni of Caltech, who said, “This is good news for planets.  Planets may live in all sorts of interesting neighborhoods that, until now, have gone largely unexplored.”  MSNBC News, Science Now and CNN provided a more balanced presentation with clear explanations of the theoretical problems this planet presents.  BBC News weighed in on the story July 18.

1Maciej Konacki, “An extrasolar giant planet in a close triple-star system,” Nature 436, 230-233 (14 July 2005) | doi: 10.1038/nature03856.
2Astronomical Unit (AU), or the mean earth-sun distance of approximately 93 million miles.
3Hatzes and Wuchteri, “Astronomy: Giant planet seeks nursery place,” Nature 436, 182-183 (14 July 2005) | doi: 10.1038/436182a.

Anomalies and constraints are good for science.  They put the brakes on speculation.  Laplace’s nebular hypothesis was a plausible-sounding naturalistic story for its day.  What would he have thought with today’s reality checks?  Puzzles and surprises are also good in that they spur the problem-solving juices of humans, who like to find out the reasons for things.  That much is good.  What turns this good astray is failing to question one’s assumptions.  None of the theorists is questioning naturalism; the idea of design is never even on the radar screen.
    The two struggling contenders for naturalistic explanations of planetary systems – core accretion and disk instability – each theory already beset by nearly insurmountable problems – have essentially been disqualified by this discovery.  It may be that the proponents will find a workaround, some convincing ad-hoc, a posteriori explanation to keep their theory in the running, but they need to be reminded that planet-building is just one local crisis within a bigger crisis about the origin of stars (05/31/2005 and 02/28/2004).  That crisis, in turn, lies within the bigger ones about the origin of galaxies (07/08/2004 and 03/03/2003) and the origin of the universe (01/23/2004 and 10/06/2004).  Exceeding them all is the crisis over the origin of life (06/16/2005 and 02/06/2005).  What do all these nested crises have in common?  Naturalism.
    The assumption that natural explanations are the only tools permissible from the toolkit of science has brought each of these investigations to a grinding halt.  To make progress, why not return to the design assumptions of Kepler, Galileo, Newton and many others?  These Christian-creationist pillars of science made great progress by believing the universe had a Designer.  Though science has mushroomed over the last two centuries in which methodological naturalism has prevailed, it’s arguable that naturalism had anything to do with the actual progress made.  The tangible, verifiable discoveries have been made with either an explicit or implicit assumption of design or purpose.  Wherever pure naturalism has led an investigation, whether in cosmology or the origin and evolution of life, it has produced little more than job security for storytellers (see 12/22/2003 entry).
    As a guiding assumption in science, naturalism is freezing up like a neglected pickup on a dusty road in the middle of nowhere.  To lubricate the engine, add the oil of intelligent design, then turn around and get back on the superhighway, back there where we got off at exit 1859.

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Categories: Astronomy

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