Tuesday, September 30, 2014

Hot Giant Planet that is Blacker than Coal

Gandolfi et al. (2014) report on the discovery of a half-Jupiter mass planet transiting an old Sun-like star every 2.7 days. This discovery combines data collected by NASA’s Kepler space telescope from 13 May 2009 to 11 May 2013 with spectroscopic follow-up observations performed with the FIES spectrograph at the Nordic Optical Telescope in La Palma, Spain. Photometric data from Kepler indicates how much starlight is blocked when the planet transits in front of its host star, allowing the size of the planet to be estimated. The FIES spectrograph measures the amount of gravitational tugging the planet has on its host star and provides the estimated mass of the planet.

Figure 1: Artists’ illustration of a hot-Jupiter orbiting a Sun-like star. Image credit: Haven Giguere & Nikku Madhusudhan.

Figure 2: Phase-folded transit light curve of KOI-183b showing the best fitting model and residuals. Gandolfi et al. (2014).

Figure 3: Radial velocity data from the FIES spectrograph with the median, 68th and 99th percentile limits. Gandolfi et al. (2014).

The planet, identified as KOI-183b, is estimated to have 0.595 ± 0.081 times the mass of Jupiter and 1.192 ± 0.052 times the radius of Jupiter. Given the mass and size of the planet, its bulk density is 0.459 ± 0.083 g/cm³. KOI-183b orbits its host star at a distance of only ~1/28th the Earth-Sun distance. As a result, KOI-183b is intensely heated and is classified as a hot-Jupiter. The radius of KOI-183b is consistent with theoretical models for heavily irradiated coreless gas-giant planets. Being so near to its host star, temperatures on KOI-183b can reach ~2000 K, hot enough to melt titanium metal.

Data from Kepler also indicates that KOI-183b periodically passes behind its host star in what is known as a secondary eclipse. The secondary eclipse signal has a depth of 14.2 ± 6.6 ppm. From the depth of its secondary eclipse signal, KOI-183b is estimated to have a very low Bond albedo of only 0.037 ± 0.019, making it one of the “darkest” gas-giant planets known so far. Basically, KOI-183b reflects only ~4 percent of the incoming radiation from its host star back into space. For comparison, that is darker than coal. Other hot-Jupiters with similarly low Bond albedos include TrES-2b and Kepler-77b.

Reference:
Gandolfi et al. (2014), “KOI-183b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]

Monday, September 29, 2014

A Highly Eccentric Brown Dwarf around a Giant Star

To date, ~10 brown dwarfs are known around giant stars (i.e. evolved stars). Brown dwarf are objects more massive than planets, but are not massive enough to count as full-fledged stars. M. I. Jones et al. (2014) report on the discovery of a brown dwarf on a highly eccentric orbit around the giant star HIP 97233. The brown dwarf, identified as HIP 97233 b, has an orbital period of 1058.8 days and a minimum mass of 20 times the mass of Jupiter.

With an orbital eccentricity of 0.61, HIP 97233 b is the brown dwarf with the most eccentric orbit known around a giant star. The mass and orbit of HIP 97233 b were both determined from the gravitational “tugging” it exerts on its host star which was observed in the form of a radial velocity signature (i.e. Doppler shifts in the star’s spectral lines).

Figure 1: Artist’s impression of a giant planet.

 Figure 2: Upper panel: Radial velocity curve for the host star of HIP 97233 b. Lower panel: Residuals from the best fit. M. I. Jones et al. (2014).

HIP 97233 b highly eccentric orbit takes it from as near as 1.0 AU to as far as 4.1 AU from its host star. M. I. Jones et al. (2014) estimate that the host star of HIP 97233 b has 1.84 ± 0.14 times the Sun’s mass and 5.20 ± 0.50 times the Sun’s radius. The host star of HIP 97233 b is considerably larger and more luminous than the Sun. At closest approach, the dayside of HIP 97233 b receives roughly 16 times the intensity of insolation as Earth receives from the Sun.

There are a number of ways through which an object like HIP 97233 b can form. Firstly, the host star of HIP 97233 b is much more massive than the Sun, enabling it to have a more massive protoplanetary disk which can allow massive planets and brown dwarfs to form more efficiently. Also, as the star evolves and swells in size, it begins to blow an enhanced stellar wind from which a giant planet can accrete a significant amount of mass and grow in mass till it reaches the brown dwarf mass regime.

The star’s high metallicity might also have enabled HIP 97233 b to form by core accretion, believed to be the main mechanism through which planets form. Finally, interaction with the protoplanetary disk before it was dissipated or with the star’s outer layers as it evolves to a giant star might have caused HIP 97233 b to migrate inward from beyond ~4 AU to where it currently is.

Reference:
M. I. Jones et al. (2014), “A planetary system and a highly eccentric brown dwarf around the giant stars HIP 67851 and HIP 97233”, arXiv:1409.7429 [astro-ph.EP]

Wednesday, August 27, 2014

A Uranus-Type Planet in a Binary Stellar System

A gravitational microlensing search by R. Poleski (2014) revealed the presence of a Uranus-type planet in orbit around a 0.6 solar mass star. The gravitational microlensing event is designated OGLE-2008-BLG-092, and the newfound planet is estimated to be ~3 times the mass of Uranus and it circles its host star at ~16 AU. For comparison, Uranus orbits the Sun at an average distance of 19 AU. This newfound planet is the first known exoplanet whose mass and orbit is similar to Uranus. The planet was detected when it and its host star fortuitously passed in front of a background star, and the gravitational field of the star-planet system magnified light from the background star.

Figure 1: Artist’s impression of a Uranus-type planet.

Planets in the Solar System can be classed into 3 groups: small rocky planets (Earth, Venus, etc), gas giants (Jupiter and Saturn) and ice-giants (Uranus and Neptune). At present, the leading methods of detecting planets around other stars (i.e. transit and radial velocity methods) have yet to turn up any extrasolar analogues of Uranus and Neptune. Such planets are far from their host stars and have orbital periods that exceed a human lifespan. As a result, both the transit and radial velocity methods have yet to turn up such planets since both methods greatly favour the detection of planets with short orbital periods. To detect extrasolar analogues of Uranus and Neptune using such methods would require exceedingly long observation timescales.

Although the technique of direct imagine can detect planets that orbit far from their host stars, this technique has so far been restricted to the detection of more massive and hotter planets that inhibit young planetary systems. These planets are very different from planets like Uranus and Neptune. At present, the only method that can detect extrasolar analogues of Uranus and Neptune seems to be gravitational microlensing as this method allows planets to be detected regardless of their orbital periods. In addition to the Uranus-type planet and its host star, the OGLE-2008-BLG-092 microlensing event also revealed the presence of a companion object in the system that is either a low mass star or a brown dwarf. In fact, the projected separation of the Uranus-type planet from its host star is only ~3 times smaller than that of the companion star (or brown dwarf).

Figure 2: Light curve of the OGLE-2008-BLG-092 microlensing event. The inset shows the planetary subevent. The presence of the companion star (or brown dwarf) is indicated by the 2010 subevent. R. Poleski (2014).

Reference:
R. Poleski (2014), “Triple Microlens OGLE-2008-BLG-092L: Binary Stellar System with a Circumprimary Uranus-type Planet”, arXiv:1408.6223 [astro-ph.EP]

Friday, August 15, 2014

Astrospheres of Evolving Massive Stars

O-type stars are amongst the most massive and most luminous stars. Isolated O-type stars that move independently through the interstellar medium have a significant influence on their surroundings from the strong stellar winds and ionizing radiation they emit. ζ Ophiuchi is a typical example of an isolated O-type star. It has ~20 times the mass and ~100,000 times the luminosity of the Sun. ζ Ophiuchi moves through the interstellar medium at ~26.5 km/s, generating a bow shock where its strong stellar wind meets the interstellar medium. In the upstream direction, the separation between the star and its bow shock, also know as the standoff distance, is ~5 trillion km, or half a light year. The enormous amount of ionization radiation emitted by ζ Ophiuchi ionizes the surrounding interstellar medium out to a radius of ~30 light years. This is ~60 times larger than the standoff distance and shows that the influence of ζ Ophiuchi extends far beyond its own stellar wind.

Figure 1: An overview of the different types of stars as well as their size and the colour with which they shine.

Massive O-type stars like ζ Ophiuchi live fast and die young. Although such stars are exceedingly rare, their immense luminosities make them easy to detect. When an O-type star begins to exhaust hydrogen in its core, it swells and transforms from a hot blue supergiant to a cooler red supergiant. All of that occurs on a timescale of only ~0.01 to 0.02 million years. Once it becomes a red supergiant, the star stops emitting ionizing radiation and its escape velocity drops dramatically. As a consequence, its stellar wind becomes slower and denser. The stellar wind during its blue supergiant phase is ~400 km/s (fast wind), while the stellar wind during its red supergiant phase slows to ~15 km/s (slow wind). Since the bow shock’s dynamical timescale of ~0.01 to 0.1 million years is much longer than the star’s evolution, a new bow shock forms around the slow wind within the relic bow shock from the fast wind. As a result, for a brief period of time, two bow shocks can exist around the star.

Figure 2: 2D simulations of the circumstellar medium at different times (indicated) for an O-type star’s evolution from blue supergiant (left-most panel) to red supergiant to the pre-supernova stage (right-most panel). The strengthening red supergiant stellar wind expands into the relic bow shock from the blue supergiant phase, creating a short-lived double bow shock. Jonathan Mackey et al. (2014).

Reference:
Jonathan Mackey et al. (2014), “Effects of stellar evolution and ionizing radiation on the environments of massive stars”, arXiv:1407.8396 [astro-ph.GA]

Thursday, August 14, 2014

A Companion Planet Keeps an Alien Earth Habitable

On Earth, the presence of tectonic activity maintains the carbon cycle and acts as a thermostat, moderating the greenhouse effect. Earth-size planets in the habitable zone are more likely to be habitable if they are tectonically active. The habitable zone is that swath of space around a star where temperatures are neither too hot nor too cold for a rocky planet to potentially sustain liquid water on its surface. However, tectonic activity is driven by internal heat. Since planets cool as they age, they will eventually have insufficient internal heat to drive tectonic activity. The demise of tectonic activity on an old, cooling planet could adversely affect the planet’s habitability. It is likely that tectonic activity would cease for Earth once it reaches an age of ~10 billion years.

A study published in the July issue of the Monthly Notices of the Royal Astronomical Society (MNRAS) shows that the gravitational pull of an outer companion planet can generate enough tidal heating for an Earth-size planet in the habitable zone to arrest its cooling. In particular, the models focused on Earth-size planets in the habitable zone of low-mass stars that are less than 0.3 times the Sun’s mass. The presence of an outer companion planet can keep the orbit of the Earth-size planet around its host star non-circular. As a result, the gravitational pull on the planet from its host star is constantly changing, potentially generating enough tidal heating to sustain tectonic activity on the planet.

Artist’s impression of a tidally-locked Earth-size planet around a low-mass star. The presence of an outer companion planet can induce sufficient tidal heating to keep the Earth-size planet warm enough to sustain tectonic activity for tens of billions of years.

The reason for the focus on low-mass stars is because such stars are much fainter than the Sun. A planet would have to be much closer to the star to receive an equivalent amount of insolation Earth gets from the Sun. This places the planet in a much stronger gravitational field, making it more susceptible to tidal heating. Furthermore, low-mass stars have extremely long lives measured in hundreds of billions to several trillion years. For comparison, the Sun has a lifespan of only about 10 billion years. The extreme longevity of low-mass stars means planets around such stars can cool below what is required to drive tectonic activity long before the stars themselves reach even a fraction of their lifespans. Also, Earth-size planets are more easily detected around low-mass stars than around more massive stars like the Sun.

The presence of an outer companion planet to an Earth-size planet in the habitable zone of a low-mass star can induce sufficient tidal heating to drive tectonic activity on the Earth-size planet for tens of billions of years or more. A Neptune-size outer companion planet would easily fulfil such a role. In fact, a substantial range of masses and orbits for the outer companion planet can induce the appropriate amount of tidal heating on the inner Earth-size planet. The least massive stars, those with ~0.1 times the Sun’s mass, are expected to live for trillions of years. Earth-size planets in the habitable zone of such stars with outer companion planets could represent the longest-lived surface habitats in the universe.

Reference:
C. Van Laerhoven, R. Barnes and R. Greenberg, “Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars”, MNRAS (July 1, 2014) 441 (3): 2111-2123.

Wednesday, August 13, 2014

Occurrence of Terrestrial Planets around Cool Stars

The ever growing number of detected exoplanets shows that small planets are far more common than larger ones. A study by Morton & Swift (2014) examines the abundance of terrestrial planets with orbital periods less than 150 days around cool stars with effective temperatures below 4,000 K (i.e. red dwarf stars). These stars make up the majority of stars in the galaxy. For comparison, the Sun has an effective temperature of 5,778 K. The study analysed data from NASA’s Kepler space telescope on exoplanets in the size range between 0.5 to 4.0 Earth radii.

Figure 1: Artist’s impression of a terrestrial planet around a cool red dwarf star.

Figure 2: Distribution of planets orbiting cool stars with orbital periods less than 150 days. The blue horizontal lines represent the standard “occurrence rate per bin” calculations. The vertical red lines represent the number of planets with a particular radius is observed. Morton & Swift (2014).

Results from the study indicate there is an average of 2.00 ± 0.45 planets between 0.5 to 4.0 Earth radii per cool star. Additionally, for planets between 0.5 to 1.5 Earth radii, there is an average of one planet per cool star. The distribution of exoplanets shows a rise with decreasing planetary radius, down to one Earth radius. Below ~0.8 Earth radii, the distribution curve appears to decrease again. If the decrease is indeed a true feature, it could mean that in the formation of terrestrial planets, only a few larger planets typically remain. As a result, planets about the size of Earth could be the most likely outcome of terrestrial planet formation.

However, the drop in abundance below ~0.8 Earth radii is most likely an artefact due to inadequate data on smaller planets since these planets are more difficult to detect (Figure 3). The true distribution is expected to keep rising below 0.5 Earth radii. The distribution of exoplanets between 0.5 to 4.0 Earth radii also indicates that planets larger than ~3 Earth radii are very rare around cool stars. Finally, estimates from the study show there are ~0.25 habitable-zone Earth-sized planets per cool star, and the number could be as high as ~0.8. This suggests that habitable-zone Earth-sized planets are ubiquitous around cool stars.

Figure 3: Discovery efficiency as a function of planet radius. Morton & Swift (2014).

Reference:
Timothy D. Morton and Jonathan Swift, “The Radius Distribution of Planets around Cool Stars”, 2014 ApJ 791 10

Tuesday, August 12, 2014

Rapidly Spinning Asteroids

Asteroids range in size from a few meters to a few hundred kilometres. Observations indicate that asteroids larger than 150 m have rotation periods longer than 2.2 hours. This is because asteroids larger than 150 m tend to be rubble-pile structures bound by gravity. A rotation period shorter than the critical 2.2 hours can cause the asteroid to break apart. In contrast, smaller asteroids are generally coherent monolithic objects, allowing them to have more rapid spin rates.

Nevertheless, two asteroids, 2001 OE84 and 2005 UW163, are both known to be larger than 150 m and have spin periods shorter than 2.2 hours. 2001 OE84 has a diameter of ~700 m and a rotation period of 29 minutes (Pravec et al. 2002), while 2005 UW163 has a diameter of ~600 m and a rotation period of 1.29 hours (Chang et al. 2014). The rotation periods of both asteroids were determined by timing the photometric variations as they spin.

2001 OE84 and 2005 UW163 are rotating too rapidly to be strengthless rubble-pile aggregates. In addition to gravity, other mechanisms such as tensile strength and cohesiveness are required to keep the asteroids from breaking apart. 2001 OE84 and 2005 UW163 could be unusually large coherent monolithic objects. Discovering more of such objects can help reveal their abundance.

Plot of diameters versus rotation period. The green and gray filled circles are objects with well determined rotation periods. The asteroids 2005 UW163 (red filled circle) and 2001 OE84 (blue filled circle) are outliers. Chang et al. (2014).

References:
- Pravec et al. (2002), “Large coherent asteroid 2001 OE84”, Proceedings of Asteroids, Comets, Meteors - ACM 2002.
- Chang et al. (2014), “A New Large Super-Fast Rotator: (335433) 2005 UW163”, arXiv:1407.8264 [astro-ph.EP]

Monday, August 11, 2014

White Dwarf Shreds Planet in a Burst of X-Rays

The globular cluster NGC 6388 is believed to harbour an intermediate mass black hole (IMBH) with ~1000 times the Sun’s mass at its centre. On 11 August 2011, the INTEGRAL satellite detected a hard X-ray transient identified as IGR J17361-4441 near the centre of NGC 6388. The hard X-ray transient was thought to have originated from the IMBH at the centre of the globular cluster. However, follow-up observations reveal that the position of the hard X-ray transient was off-centre and could not possibly be related to the IMBH.

Figure 1: Artist’s impression of a planet in the dense stellar environment of a globular cluster.

A study by M. Del Santo et al. (2014) suggests that IGR J17361-4441 is consistent with a tidal disruption event (TDE). Its nature as a TDE is based on two pieces of empirical evidences. Firstly, the decline of the X-ray light curve is typical for a TDE (Figure 2). Secondly, observations of TDEs from the disruption of stars by supermassive black holes (SMBHs) show a thermal emission component that does not evolve significantly with time. Such a thermal emission was observed for IGR J17361-4441 (Figure 3).

Observational analysis and theoretical calculations indicate that IGR J17361-4441 is most likely a TDE involving a white dwarf and a disrupted planetary object. The mass of the disrupted planetary object depends on the accretion efficiency (i.e. how much of the planetary material that is accreting onto the white dwarf is converted into energy to power the hard X-ray transient). It turns out that the disrupted planetary object is estimated to have a mass that is on the order of one-third the Earth’s mass.

The TDE occurred when this free-floating planetary object came too close to the white dwarf. Nevertheless, the rate of such a TDE in a globular cluster is unknown since the densities of white dwarfs and free-floating planets in globular clusters are uncertain. It is reasonable to assume that free-floating planets are more common in globular clusters as the dense stellar environment makes it more likely for planetary systems to be perturbed by passing stars. Assuming such a TDE occurs in a given globular cluster at a rate of one every ~3,000 years and the total number of globular clusters in the Milky Way is roughly 150, then the total event rate is once per ~20 years.

Figure 2: Observed X-ray light-curve of IGR J17361-4441 fitted with a model expected from a TDE (red line). M. Del Santo et al. (2014).

Figure 3: Observed thermal emission component of IGR J17361-4441. M. Del Santo et al. (2014).

Reference:
M. Del Santo et al. (2014), “The puzzling source IGR J17361-4441 in NGC 6388: a possible planetary tidal disruption event”, arXiv:1407.5081 [astro-ph.HE]

Sunday, August 10, 2014

Swarm of Planets Circling a Supermassive Black Hole

A supermassive black hole (SMBH) with ~4 million times the Sun’s mass sits in the centre of the Milky Way. Over the decades, observations have revealed the presence of a group of stars known as the S-stars that orbit very close to the SMBH. The presently known S-stars are stars that are more massive and more luminous than the Sun. This indicates a larger population of fainter members that continue to elude detection. The S-stars zip around the SMBH with speeds of up to ~10,000 km/s. Currently, S0-102 holds the record for being the star with the shortest known orbital period around the SMBH at the galaxy’s centre. S0-102 has an orbital period of 11.5 years.


The S-stars are believed to have originated from binary star systems that are disrupted due to close passages near the SMBH. When a binary star system is disrupted, one star can get ejected from the vicinity of the SMBH, while the other star is left behind in a tight orbit around the SMBH. Since planets are ubiquitous around stars, it is reasonable to assume that these disrupted binary star systems have planets of their own, at least before they became disrupted. One study by Ginsburg, Loeb & Wegner (2012) show that such disruption events can strip planets from their host stars. These planets are either left behind on independent orbits around the SMBH or ejected away from the SMBH as hypervelocity planets. Over time, planets that are left behind can create a swarm of planets around the SMBH.

Planets can attain terrific speeds if their orbits that take them sufficiently close to the SMBH. For example, a rocky planet with an Earth-like composition passing as close as it can to the SMBH without being tidally torn apart can reach up to several percent the speed of light. A head-on collision with even a small object at such a speed would be devastating for the planet. One study by Nayakshin, Sazonov & Sunyaev (2011) suggests that given the right conditions, a fragmentation cascade could destroy a swarm of planets around a SMBH. When an asteroid collides with a planet at very high velocity, it can shatter the planet, creating more fragments that can collide with more planets and so on. Such a process of fragmentation cascade could grind a swarm of planets around a SMBH into high velocity dust.

References:
- Ginsburg, Loeb & Wegner (2012), “Hypervelocity Planets and Transits Around Hypervelocity Stars”, arXiv:1201.1446 [astro-ph.GA]
- Nayakshin, Sazonov & Sunyaev (2011), “Are SMBHs shrouded by "super-Oort" clouds of comets and asteroids?”, arXiv:1109.1217 [astro-ph.CO]

Saturday, August 9, 2014

Hypervelocity Planets

At the centre of the Milky Way sits a supermassive black hole (SMBH) with a mass of around 4 million times the mass of the Sun. A close encounter of a binary star system with the SMBH can cause one star to be ejected as a hypervelocity star (HVS) with sufficient velocity to escape the gravitational pull of the Milky Way, while the other star becomes captured into an eccentric orbit around the SMBH. This is the most likely mechanism for the production of HVSs. The existence of HVSs was first theorised in 1988 and the first HVS was discovered in 2005. Several HVSs have been discovered since. Ordinary stars in the galaxy have velocities on the order of ~100 km/s, while HVSs have velocities on the order of ~1000 km/s.

A study by Ginsburg, Loeb & Wegner (2012) investigates what happens when a binary star system hosting a planetary system gets disrupted by the SMBH at the centre of the Milky Way. In particular, the study examines the generation of hypervelocity planets (HVPs). The possible outcomes from such an interaction are - a HVS, one or more HVPs, a HVS with one or more bound planets, a star left behind in an orbit around the SMBH with one or more bound planets, a planet collides into its host star, or one or more planets left behind in independent orbits around the SMBH (Figure 2 & 3).

Figure 1: Artist’s impression of a lone planet in the outskirts of a galaxy.

Figure 2: The panel illustrates the possible outcomes after a binary star system with 4 planets is disrupted by the SMBH at the Milky Way’s centre. After binary disruption a HVS is produced with two bound planets. The second star remains in a highly eccentric orbit around the SMBH. The second star’s planets are removed, and the first planet falls into a highly eccentric orbit close to the SMBH, while the second planet is ejected into a much larger, but also highly eccentric orbit around the SMBH. Ginsburg, Loeb & Wegner (2012).

Figure 3: The panel illustrates the possible outcomes after a binary star system with 4 planets is disrupted by the SMBH at the Milky Way’s centre. After binary disruption a HVS is produced with two bound planets. The second star remains in a highly eccentric orbit around the SMBH. The second star’s planets are both ejected as HVPs. Ginsburg, Loeb & Wegner (2012).

The study draws on a few sets of simulations. One particular set of simulations involves binary star systems with two planets (i.e. one planet per star). 1000 simulation runs were performed in this set of simulations. The mass of each star is set at 3 times the Sun’s mass, comparable to the masses of the presently known HVSs. The initial separation between both stars in the binary star system is 0.2 AU, while the star-planet separation varies with uniform probability in the range 0.02 to 0.04 AU. The simulation runs show that the average HVS velocity is ~1500 km/s, while the average HVP velocity is ~3000 km/s (Figure 4). The velocities of HVPs are on average ~1.5 to 4 times the velocities of HVSs. Another set of simulation runs involving binary star systems with four planets (i.e. two planets per star) produce qualitatively similar results.

For a binary star system with two planets, the probability of producing a HVP is ~30 to 40 percent. For a binary star system with four planets, the probability increases to ~70 to 80 percent. The velocity distribution of HVPs reveals a small number of HVPs with exceptionally high velocities of ~10,000 km/s (Figure 4). These extreme outliers speed through space at a few percent the speed of light. A HVP travelling at a speed of 10,000 km/s would traverse a distance of one light year in just 30 years. An observer on such a planet can see the constellations change in a matter of years while the planet is travelling through the galaxy. As the planet speeds out of the galaxy, the Milky Way would appear as a receding disk of light. Such a planet is destined to travel through the immense intergalactic void separating galaxies and clusters of galaxies.

Figure 4: Velocity distribution of HVSs and HVPs. This sample comes from 1000 simulation runs involving a binary star system with two planets. Ginsburg, Loeb & Wegner (2012).

“These warp-speed planets would be some of the fastest objects in our galaxy. If you lived on one of them, you’d be in for a wild ride from the centre of the galaxy to the universe at large,” said astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and co-author of the study. “Other than subatomic particles, I don’t know of anything leaving our galaxy as fast as these runaway planets,” added lead author Idan Ginsburg of Dartmouth College.

Reference:
Ginsburg, Loeb & Wegner (2012), “Hypervelocity Planets and Transits Around Hypervelocity Stars”, arXiv:1201.1446 [astro-ph.GA]