Jovian planet in Globular Cluster M Calm bystander in stellar drama

Jovian planet forms around Sun-tike star in outskirts Of M4 13 billion years ago.

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Planet system travels to core of Globular Cluster M4

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Star passes through M4's core and Is drawn toward a neutron star and its companion.

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Slowly spinning neutron star captures star and planet; its original partner Is ejected into space.

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Sun-like star swells toa red giant, spilling matter onto neutron --

I / Materat from \y re^ giant . transfers to neutron star.

Neutron star "spins up,* becoming a pulsar that spins 100 times a second (PSR 6162D-26). Red giant becomes a helium white dwarf. —a

Jovian planet continues to orbit, relatively undisturbed, around new binary system.

Pulsar /) White dwarf B1620-26 + / *

Fig. 8.2. A conceptual schematic of one of the proposed exchange formation mechanisms for the planet around PSR B1620-26 in the globular cluster Messier 4. (Adopted from a Space Telescope Science Institute/NASA press image, http://hubblesite.org/newscenter/archive/releases/2003/2003/19/)

Fig. 8.2. A conceptual schematic of one of the proposed exchange formation mechanisms for the planet around PSR B1620-26 in the globular cluster Messier 4. (Adopted from a Space Telescope Science Institute/NASA press image, http://hubblesite.org/newscenter/archive/releases/2003/2003/19/)

planets formed around the pulsar, such as for PSR B1257+12. The latter are in close circular orbits, and the planets are of relatively low mass; by contrast exchanges tend to slightly favour the most massive planets, and will lead to large, eccentric orbits.

Coincidentally, at the Caltech workshop in 1992, Backer reported anomalous timing residuals for PSR B1620-26 in the metal poor globular cluster Messier 4, the second millisecond pulsar discovered in a globular cluster. Unlike PSR B1257+12, PSR B1620-26 is a binary pulsar, orbited by a low mass white dwarf, the remnant of the star which spun the pulsar up to its current millisecond period. The timing residuals were consistent with a Jovian mass planet in a distant orbit around the pulsar, but did not preclude other explanations (Thorsett et al., 1993). A number of contending models were proposed to explain the system, (e.g., Sigurdsson, 1993; Joshi & Rasio, 1997), and in 1999 additional data strongly constrained the system, requiring the presence of a low mass companion (Thorsett et al., 1999), while new modeling explaind the detailed kinematics of the system more adequately (Ford et al., 2000).

Then, in 2003, at a workshop at the Kavli Institute in Santa Barbara, it was realised that the Hubble Space Telescope had serendipitously imaged the location of the pulsar, and its stellar companion, a low mass white dwarf, was observed. This provided two new pieces of information, a "cooling age" for the system, showing that the white dwarf had formed from its parent star about 500 million years earlier, and an independent constraint on the inclination of the pulsar-white dwarf orbital plane, which in turn allowed independent constraints on the other mystery companion. This is, in fact, a roughly 2 Jupiter mass planet, in a moderately eccentric orbit with an orbital period of about 100 years. Since then, the planet orbit has been observed to cross the periastron point, reversing the sign of its gravitational perturbation on the star, confirming the orbital parameters (Stairs, private communication). The planet is generally thought to have been exchanged into the current system, having originally formed around a ~ 0.85 solar mass star in a fairly wide orbit, some 12.7 billion years ago. Currently there are two competing exchange models, one in which the planet was exchanged into the system when the binary formed (Sigurdsson, 1993), the other conjecturing that the binary pulsar formed first, with the planet exchanged in an independent encounter sometime later (Fregeau et al., 2006). Alternative formation scenarios have been proposed, (e.g. Beer et al., 2004), but they have a hard time accounting for the detailed orbital parameters of the system, in particular the high orbital inclination of the planet relative to the plane of the inner white dwarf orbit.

If the PSR B1620-26 system formed through exchange with a main sequence star, then planet formation started very early in the history of the universe. It is difficult to explain how a giant planet could form in such a metal poor system, and it is possible that the system provides evidence for a second planet formation process.

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