Planets in exotic Locations

Exotic I: Planets around Pulsars

Main sequence lifetime ~ 10 million years Helium burning ~ 1 million years Carbon burning ~ 300 years Oxygen burning ~ 2/3 year Silicon burning ~ 2 days The Progenitors to Pulsars: Exploding Massive stars The burning stages of massive stars After Si burning the core collapses resulting in a supernova explosion. What is left behind is a neutron star. These are Type II supernova

The „Lighthouse“ Beacons of Pulsars

Properties of Neutron Stars (Pulsars) Progenitor Mass: 8-20 M sun Remnant mass: < 3 M sun (otherwise it becomes a black hole) Pressure support: Neutron degeneracy pressure Radius: ~10 km Density: 200 million tons/cm 3 Magnetic field strength: ~ Gauss Periods: 1.5 millisecs to 8.5 s Rotation period of B : P = ± secs These are very stable clocks! => timing method

A shaky start: Nature 1991

1992: initial position of the pulsar used in the barycentric motion of the Earth was off by 7 arcmin They detected the eccentricity of the Earth’s orbit

The (Really) First Exoplanets: in 1992 Arecibo Radio-telescope

98 d orbit removed, 66 d orbit remains 66 d orbit removed, 98 d orbit remains

PSR system: Planet A: M = 0.02 M_Earth P = 25.3 d ; a = 0.19 AU Planet B: M = 4.3 M_Earth P = 66.5 d ; a = 0.36 AU Planet C: M = 3.9 M_Earth P = 98.2 d ; a = 0.46 AU fourth companion with very low mass and P~3.5 yrs Interaction between B & C Confirms the planets and Establishes true masses!

Planets C & D near 3:2 resonance => interact gravitationally! Rasio et al These effects were observed in PSR (Wolszczan 1994)  Very strong evidence that this planetary system is real!

Pulsar with a 0.3 Msun mass companion in a 191 d orbit After removing the timing variations of the stellar companion there are additional variations in the residuals

Phinney 1993: Period variations due to planet Mearth with P > 15 yrs. Thorsett et al : Variations are consistent either with a planet at ~10 AU, or a star at ~50 AU orbit This planet is uncertain. Currently there is only one pulsar with planetary companions

Origin of the Pulsar Planets 1.First Generation Planets: These „rocks“ are remnants of planets (maybe giant planets) that survived the supernova explosion 2.Second Generation Planets: Planets that formed in the debris disk left behind after the supernova explosion (more likely) Debris disk found around another pulsar fits this picture! Unfortunately, we only have one example of a pulsar with planets, until we find more such systems the nature of pulsar planets will be unknown.

Exotic II: Pulsating white dwarfs & sdB stars and eclipsing WD binaries (NN Ser & DP Leo)

We Can Observe White Dwarfs for Timing But first, what is a White Dwarf? The geriatric remains of sun-like stars All stars with masses less than 8M  will become white dwarf stars

The Timing Method With White Dwarf Stars White Dwarf stars are extremely small (radius ~ Earth) Their mass is comparable to the sun, so they are extremely dense Radial velocity impossible on such broad spectral lines Observing planetary transits is improbable Imaging studies have so far been unsuccessful White Dwarf stars provide a few different stable “clocks” Binary White Dwarf stars = eclipses Pulsating White Dwarf stars = time of arrival of pulses WD

Optical Light Curves of ZZ Ceti Stars Searching for Planets Around Oscillating White Dwarfs Mullally et al. (2008, ApJ, 676, 573) looked at a sample of 15 pulsating white dwarfs

One white dwarf, GD 66 looks promising: Arrival time variations consistent with a ~2 M Jup companion in a 4.5 year orbit…but one has to be careful: 1.Evolutionary changes can cause period changes 2. Unstable modes can cause period changes 3. Beating of modes can cause period changes (WD stars tend to be multiperiodic pulsators). But the amplitude of the mode shows variations

Subdwarf B Stars (sdB) sdB stars are believed to be core He-burning stars of 0.5 M on the extended horizontal branch that have lost their envelope T eff ~ – K Periods 100 – 250 secs V391 Peg

sdB stars sdB stars in the HR diagram:

O-C for two pulsation frequencies look the same

Sub-stellar Objects in Strange Places: The Sub-stellar companion to the sdB star HD found with traditional radial velocity variations A 8-20 M Jup mass object in a 2.9 d orbital period…so why is this interesting? P = 2.9 days → a = 0.05 AU (assuming a 2 solar mass star) = 10 solar radii. On giant branch: Stellar radius R sun At one point this companion was in the envelope of the star!

Planets around the cataclysmic eclipsing binary NN Ser White Dwarf: Mass: Solar masses Temperature = K Orbital Period 3.12 hours M4 Dwarf companion: Mass: 0.11 Solar masses Temperature ~ 3000 K NN Ser is an eclipsing system. If there are additional companions around one or both stars this will change the expected time of the eclipse. Mass transfer Not to scale!

UT Case Study: NN Serpentis Parsons et al. 2010, MNRAS Eclipses, which occur every hr, are the “clock” We observe this clock for any deviations (for example, say we see hr between eclipses, or hr)

One planet fit to the variations in the eclipse timing: Two planet fit: P 1 = 15.5 years M 1 = 6.9 M Jup P 2 = 7.7 years M 2 ~ 2.2 M Jup 2:1 resonance

These planets have to be circumbinary planets: Formation scenarios: First or Second Generation planets First Generation: Planets formed with stars, but these would have to have survived the supernova explosion Second Generation: Planets formed after the common envelope phase Are they planets at all? Dynamical stability in question! X

Planets around the cataclysmic eclipsing binary DP Leo White Dwarf companion with Mass > 1 Solar mass Companion is a cool star that is transfering mass to the W.D. Orbital Period = 89 minutesDP Leo is a post Common Envelope star (CE). During the evolution of the W.D. the secondary was in the envelope of the companion star.

Exotic III: Planets in globular and open clusters

HST transit search for hot Jupiters in 47 Tuc Only 120 light years across!

Targets: 47 Tuc Main Sequence Stars (number of usable targets ~34000) Telescope Time Allocation: 8.3 X 24 hours, continuous observation (120 X 96.4-min HST Orbits) Observations schedule: 1999 July 3-11 Data Obtained: 636 F555W Images (exposure time: 160 sec per image) 653 F814W Images (exposure time: 160 sec per image) Number of Planetary Transits expected: about 1% of Main Sequence Stars of the Solar Neighbourhood host a Short-Period Planet ("Hot Jupiters") Transit Probability for the Hot Jupiters: ~ 10% Therefore 1 Transit/1000 Stars is expected Transits for the full surveyed Stellar Sample are expected if the 47 Tuc Planet occurence is the same as in Field Stars Results: No Planetary Transit Detected But: dynamical environment and [Fe/H]=-0.7! HST transit search for hot Jupiters in 47 Tuc

An ancient planet in M4 orbiting a pulsar/WD binary Timing residuals in pulsar PSR B reveal a high-mass companion Additional residuals point toward a 3 rd body, could be planet orbiting both stars at large separation. HST detected 2 nd body => WD Assuming co-planarity 3 rd body = planet with ~2.5 Mjup with P~100yrs and age of 12.7 Gyrs!

The Hyades

Hyades stars have [Fe/H] = 0.2 According to V&F relationship 10% of the stars should have giant planets, The Hyades Paulson, Cochran & Hatzes surveyed 100 stars in the Hyades According to V&H relationship should have found 10 planets But found zero planets! Something is funny about the Hyades. Is it their youth (650Myr) and activity level?

HD shows the same period in in photometry, color, and activity indicators. This is not a planet! In 1996 Michel Mayor announced at a conference in Victoria, Canada, the discovery of a new „51 Peg“ planet in a 3.97 d. One problem… The problem with active stars:

Exotic IV: HD Ab : an impossible Planet?

Konacki et al. 2005: An extrasolar giant planet in a close triple-star system (aka the Tatooine planet search) HD A & B: A planet ?? m sin i = 1.14 Mjup Another 3 rd star: P = 156 d P~26 yrs P=3.4 d This would be extremely interesting as protoplanetary disk was truncated at ~1.3 AU, well inside the ice-line! Capture scenario for 2 nd star?

Eggenberger et al. 2007: No evidence for HD Ab!

Exotic V: HIP b: a planet from outside the Milky Way?

Setiawan et al. (2010): HIP 13044, a highly evolved star, is orbited by a giant planet with P=16.2 days J Setiawan et al. Science 2010;330:

..and belongs to the Helmi stellar stream that consists of stars that originally belonged to a dwarf galaxy that merges with the Milky Way: HIP is extremely metal-poor!

We know little about about the RV behavior of this type of star. HIP shows signs of variability of its spectral lines: I’m not convinced this is a real planet….

Exotic Planets Summary: A multi-planet system around a milli-second pulsar Candidates around pulsating white dwarfs (GD 66), one sdB star (V 391 Peg) and eclipsing WD systems (NN Ser & DP Leo) No planets in globular cluster 47 Tuc and open cluster Hyades, one candidate in globular M4 “Impossible” planet HD Ab is not real HIP b, a giant planet of extragalactic origin?