Planets in exotic Locations
Exotic I:
Planets around Pulsars
The Progenitors to Pulsars: Exploding Massive stars
The burning stages of
massive stars
Main sequence lifetime ~ 10 million years
Helium burning ~ 1 million years
Carbon burning ~ 300 years
Oxygen burning ~ 2/3 year
Silicon burning ~ 2 days
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 Msun
Remnant mass: < 3 Msun (otherwise it becomes a black hole)
Pressure support: Neutron degeneracy pressure
Radius: ~10 km
Density: 200 million tons/cm3
Magnetic field strength: ~ 1013 Gauss
Periods: 1.5 millisecs to 8.5 s
Rotation period of B1937+21:
P = 0.0015578064924327 ±0.0000000000000004 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 1257+12 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!
These effects were observed in
PSR 1257+12 (Wolszczan 1994)
Very strong evidence that
this planetary system is real!
Rasio et al. 1992
•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 14-400 Mearth with P > 15 yrs.
• Thorsett et al. 1993 : 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
Searching for Planets Around Oscillating White Dwarfs
Optical Light Curves of ZZ Ceti Stars
Mullally et al. (2008, ApJ, 676, 573) looked at a sample of 15
pulsating white dwarfs
One white dwarf, GD 66 looks promising:
But the amplitude of the
mode shows variations
Arrival time variations consistent with a ~2 MJup 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).
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
• Teff ~ 22.000 – 40.000 K
• Periods 100 – 250 secs
V391 Peg
sdB stars in the HR diagram:
sdB stars
O-C for two pulsation frequencies look the
same
Sub-stellar Objects in Strange Places: The Sub-stellar companion to the sdB
star HD 149382 found with traditional radial velocity variations
A 8-20 MJup 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 10-50 Rsun
At one point this companion was in the envelope of the star!
Planets around the cataclysmic eclipsing binary NN Ser
Not to scale!
Orbital Period 3.12 hours
White Dwarf:
Mass: 0.535 Solar masses
Temperature = 37000 K
Mass transfer
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.
UT Case Study: NN Serpentis
• Eclipses, which occur every 3.120 hr, are the “clock”
• We observe this clock for any deviations (for example, say
we see 3.119 hr between eclipses, or 3.126 hr)
Parsons et al. 2010, MNRAS 402 2591
One planet fit to the variations in
the eclipse timing:
P1 = 15.5 years
M1 = 6.9 MJup
2:1 resonance
Two planet fit:
P2 = 7.7 years
M2 ~ 2.2 MJup
These planets have to be circumbinary planets:
X
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!
Planets around the cataclysmic eclipsing binary DP Leo
Orbital Period = 89 minutes
White Dwarf companion with
Mass > 1 Solar mass
Companion is a cool star
that is transfering mass to
the W.D.
DP 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!
HST transit search for hot Jupiters in 47 Tuc
• 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
• 30-40 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!
An ancient planet in M4 orbiting a pulsar/WD binary
•Timing residuals in pulsar
PSR B1620-26
reveal a high-mass companion
• Additional residuals point toward
a 3rd body, could be planet orbiting
both stars at large separation.
• HST detected 2nd body => WD
Assuming co-planarity 3rd body
= planet with ~2.5 Mjup with
P~100yrs
and age of 12.7 Gyrs!
The Hyades
The Hyades
• Hyades stars have [Fe/H] = 0.2
• According to V&F relationship 10% of the stars should have giant planets,
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?
The problem with active stars:
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…
HD 166435 shows the
same period in in
photometry, color, and
activity indicators.
This is not a planet!
Exotic IV:
HD 188753 Ab : an impossible Planet?
Konacki et al. 2005: An extrasolar giant planet in a close triple-star system (aka the
Tatooine planet search)
HD 188753 A & B:
A planet ??
m sin i = 1.14 Mjup
P=3.4 d
P~26 yrs
Another 3rd star: P = 156 d
This would be extremely interesting as protoplanetary disk was truncated at ~1.3 AU,
well inside the ice-line! Capture scenario for 2nd star?
Eggenberger et al. 2007: No evidence for HD 188753 Ab!
Eggenberger et al. 2007: No evidence for HD 188753 Ab!
Eggenberger et al. 2007: No evidence for HD 188753 Ab!
Exotic V:
HIP 13044 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;330:1642-1644
HIP 13044 is extremely metal-poor!
..and belongs to the Helmi stellar stream
that consists of stars that originally belonged to
a dwarf galaxy that merges with the Milky Way:
We know little about about the
RV behavior of this type of star.
HIP 13044 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 188753 Ab is not real
• HIP 13044 b, a giant planet of extragalactic origin?