Monthly Archives: July 2016

Does Planet Nine Solve the Riddle of the Sun’s Obliquity?

The sizeable tilt of the proposed Planet Nine body could explain other unexplained features of the solar system as well as the observed clustering of extended scattered disk object beyond the Kuiper Belt.  The Caltech astrophysics team who introduced the world to Planet Nine in January (1) think it may also explain the Sun’s six degree tilt with respect to the plane of the ecliptic (2).  In addition, the presence of a distant, sizeable Planet X object, whose closest approach to the Sun is argued to be 250 Astronomical Units away (3), could be affecting the tilt of the entire planetary system orbiting the Sun.

“Using an analytic model for secular interactions between Planet Nine and the remaining giant planets, here we show that a planet with similar parameters can naturally generate the observed obliquity as well as the specific pole position of the sun’s spin axis, from a nearly aligned initial state. Thus, Planet Nine offers a testable explanation for the otherwise mysterious spin-orbit misalignment of the solar system.” (3)

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New Dwarf Planet Sparks Debate about Planet Nine

The family of extended scattered disk objects beyond the classical Kuiper Belt just keeps getting bigger.  The latest addition to this population of objects is a fairly substantial dwarf planet 700km across, currently referred to by the moniker 2015 RR245 (1).  Its elliptical orbit is not absolutely defined as yet, but the best estimates give it an aphelion distance of about 120 Astronomical Units, and a closest approach to the Sun of about 34 AU (2).  The Minor Planet Center describes the object as the 18th largest in the Kuiper Belt, but it is not yet clear what its surface features might include.  2015 RR245 takes approximately 700 years to orbit the Sun.  2015 RR245was discovered by National Research Council of Canada’s Dr J.J. Kavelaars while studying images taken by Canada–France–Hawaii Telescope in Hawaii in September 2015 .

Does this object fit in with Mike Brown’s analysis of the cluster of 6 (now 7) Sednoid objects which he argues (along with his dynamicist colleague Konstantin Batygin) point to the existence of a substantial planet beyond the Kuiper Belt (3)?  Given the vague data regarding the orbit of 2015 RR245, it is perhaps too early to say.  But other scientists are already citing the on-going discoveries of distant objects like 2015 RR245 as reasons to be cautious.  In an informative on-line article, more nuanced than its title suggests, astrophysicist Ethan Siegel notes that the pattern of discovery of objects within and beyond the Kuiper Belt is subject to an observational bias favouring the closest objects.  This means that the unknown populations of objects yet to be discovered may eventually statistically overwhelm the small populations of extended scattered disk objects and Sednoids already discovered.  This, he argues, could bring Brown and Batygin’s analysis of the cluster into question. 

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Interstellar Planet Formation

Continuing the discussion from last month’s blog about planetessimal-building conditions in space beyond the solar system’s heliopause boundary (1).  In my February paper, I discussed anomalous results which had come back from various space probes regarding the influx of large grain interstellar dust into the heliosphere (2).  More on this in a moment.  A correspondent of mine had noted similarities between what I had been writing about and previous work by Paul LaViolette, who had written about the origins of the dust picked up by the Ulysses spacecraft:

“I would suggest that the dust originates from a circumsolar dust sheath that is concentrated toward the plane of the ecliptic in a fashion similar to the disk girdling the star Beta Pictoris and that is co-moving with the Sun. Infrared observations confirm the existence of dust sheaths around other stars in the solar neighborhood, leading to the conclusion that our Solar System is similarly shrouded.” (3)

The 20 million year old star Beta Pictoris provides astronomers with the best example of a gas giant exoplanet found orbiting within an evolving proto-planetary disk, made all the more dramatic by its side-on view and the brightness of scattered light from the revolving disk:

“In 1984 Beta Pictoris was the very first star discovered to host a bright disc of light-scattering circumstellar dust and debris. Ever since then Beta Pictoris has been an object of intensive scrutiny with Hubble and with ground-based telescopes. Hubble spectroscopic observations in 1991 found evidence for extrasolar comets frequently falling into the star.” (4)

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4Mj Dark Star found in Triple Star System

A young ‘Dark Star’, weighing in at 4 Jupiter masses, is one of only a few such exoplanets to have been directly imaged.  It’s also a rather curious object for another reason:  It’s orbiting the main star of a triple star system some 340 light years away, in a dynamical arrangement which lies on the very edge of mathematical possibility (1).  HD131399ab is just 16 million years old, and could be classified as an ultra cool sub-brown dwarf rather than a Jovian class gas giant.  At this youthful age its temperature is about 600 degrees Celsius, allowing it to be directly imaged in infra-red by SPHERE operated by the European Southern Observatory.

The triple star system is indeed curious.  The two minor stars (B and C) orbit the main star A at a distance of about 300 Astronomical Units, all the time twirling around each other at approximately Saturn’s distance from the Sun.  The newly discovered exoplanet, HD131399ab, also orbits around the main star A in a wide orbit “about twice as large as Pluto’s if compared to our solar system, and brings the planet to about one-third of the separation of the stars [B & C] themselves.” (2).  The massive planet’s orbit around its parent star is by far the widest known orbit within a multi-star system.

This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines). The planet orbits the brightest star in the system, HD 131399A.

This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines). The planet orbits the brightest star in the system, HD 131399A.  Image Credit ESO

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