Antimatter-Driven Fusion Propulsion

Every generation has the obligation to free men's minds for a look at new worlds . . . to look out from a higher plateau than the last generation.

- Ellison S. Onizuka

Inertial Confinement Fusion (ICF) is a process where nuclear fusion reactions are initiated by heating and compressing a spherical fuel pellet. In conventional ICF, energy is delivered to the outer layer of the fuel pellet using high energy beams of lasers, electrons or ions. This causes the outer layer of the fuel pellet to explode outwards and produce a reaction force which causes the remainder of the fuel pellet to implode inwards. The implosion process launches a powerful shockwave into the center of the fuel pellet which violently compresses and heats the fuel to the point where nuclear fusion occurs. This drives a thermonuclear burn wave which propagates out from the center of the fuel pellet and consumes it. Within the thermonuclear burn wave, energy is generated when light isotopes such as deuterium and tritium fuse into heavier ones.

A number of studies have been done to investigate the use of conventionally driven ICF as a means of propulsion for space travel. However, conventional ICF drivers are too massive and complex for use as a means of space propulsion. They also tend to require a large amount of recirculating power, waste heat radiators and large lasers or ion-beam particle accelerators. In comparison, antimatter-driven ICF offers zero need for recirculating power, very low antiproton injection energy and a vast reduction in equipment mass and waste heat generators. The energy unleashed from the annihilation of antiprotons with protons is ultimately used to drive an implosion within the spherical fuel pellet and the vast majority of the propulsion energy will be generated from the fusion process itself.

When an antiproton enters normal matter, it slows down by transferring its kinetic energy to nearby electrons in a process known as electron drag. Once it has slowed down sufficiently, the antiproton will displace an outer orbital electron of an atom. The antiproton then cascades towards the ground state via the emission of X-rays and settles into a stable close orbit around the atomic nucleus. From here, the antiproton annihilates with either a proton or neutron from within the nucleus and the annihilation energy released from each annihilation event is about 1.88 GeV.

The annihilation of an antiproton with a proton produces on average 2 neutral pions and 3 charged pions. Each neutral pion decays into 2 energetic gamma ray photons while each charged pion decays into a muon and a neutrino. Subsequently, each muon decays into an electron, an electron-antineutrino and a muon-neutrino. The annihilation of protons by antiprotons generally produces an equal number of positive and negative pions. On the other hand, the annihilation of neutrons by antiprotons produces a greater number of negative pions than positive pions and the proportion of charged pions to neutral pions is also greater.

To describe the process of antimatter-initiated ICF, it is assumed that the fusion fuel is comprised entirely of deuterium and tritium. Both deuterium and tritium are isotopes of hydrogen and the combination of deuterium and tritium as a fusion fuel is simply denoted as DT. The nucleus of a deuterium isotope consists of one proton and one neutron while the nucleus of a tritium isotope consists of one proton and two neutrons. The fusion of one deuterium nucleus with one tritium nucleus produces a single helium-4 nucleus and one neutron.

As shown in the diagram above, the antiproton-driven ICF spherical fuel pellet configuration basically consists of an outer spherical DT ablator shell surrounding a solid spherical DT fuel shell. DT gas fills the volume within the solid DT fuel shell. The DT ablator shell is seeded with uranium-238 (U-238) to enhance the deposition of annihilation energy. The DT ablator shell and the solid DT fuel shell are each made up of a few milligrams of fusion fuel, and the entire antiproton-driven ICF spherical fuel pellet measures only a few millimeters in diameter.

The process begins when a spherical omni-directional beam of antiprotons is directed at the fuel pellet and the kinetic energy of the antiproton beam is precisely controlled so that the antiprotons annihilate only within the DT ablator shell. Almost all of the antiprotons will annihilate with either a proton or neutron of the U-238 nuclei seeded throughout the DT ablator shell. When an antiproton annihilates with a U-238 nucleus, a fraction of the annihilation energy gets transferred to the nucleus through pion interactions. This causes the nucleus to break up violently and produce highly-ionizing short-range nuclear fragments.

Nuclear fragments from the fission of U-238 deposit energy into the DT ablator shell. This causes the shell it to heat up and expand outwards with velocities of several tens of km/s. By the conservation of momentum, the rapid expansion of the DT ablator shell causes the remaining portion of the spherical fuel pellet to violently implode which intensely compresses and heats the DT fuel. The kinetic energy of the DT fuel shell at peak velocity is about a few hundred thousand joules.

When the imploding DT fuel shell stagnates on the DT gas filling the central cavity within the DT fuel shell, all of its kinetic energy will be converted to pressure within the DT gas. At this stage, there will be a high temperature but low density hotspot containing a few percent of the DT fuel mass being created at the center of the DT fuel assemblage. Surrounding the hotspot will be the bulk of the DT fuel which is at a lower temperature but higher density. At this instance, approximately half of the energy will be contained in the central hotspot while the rest of the energy will be compressional energy contained within the surrounding DT fuel.

The shockwave launched by the implosion converges on the central hotspot and violently heats it up to fusion conditions. Thermonuclear fusion begins when the temperature of the hotspot exceeds about a hundred million degrees Centigrade. A fusion chain reaction occurs and drives a thermonuclear detonation burn wave that propagates radially outwards from the hotspot. This thermonuclear burn wave consumes a significant fraction of the entire DT fuel assemblage and generates a fusion yield of a few hundred million joules for each DT fuel pellet.

Credit: David Robinson (2007)

For a spacecraft in reality, this entire process must be repeated at a frequency of a few tens of hertz to provide continuous thrust. Antiproton-driven ICF can generate propulsion exhaust velocities of a few thousand kilometers per second. Each ICF target requires only tens of picograms of antiprotons to initiate nuclear fusion, whereby one gram is a trillion picograms. The estimated mass of antiprotons that is required to initiate and sustain nuclear fusion to accelerate a 100 ton spacecraft to a velocity of 1000 km/s is only a few grams. Such a propulsion system will be useful for the exploration of the outer solar system, Kuiper belt and beyond. At a velocity of 1000 km/s, it only takes 173 days to travel a distance of 100 AU, where one AU is the average Earth-Sun distance. Nevertheless, the capabilities of antiproton-driven ICF still falls short of what is required for traveling to the nearest stars within reasonable durations. Traveling at 1000 km/s, it will take almost 1300 years to get to the nearest star - Proxima Centauri.

To conclude, before antimatter-driven ICF can be utilized as a means of propulsion for interplanetary space travel, there are still numerous technological challengers to overcome. The production, collection, storage, handling and precision beaming of antiprotons will present significant technical challenges. Nonetheless, the potentials and benefits of using antimatter-driven ICF for space propulsion or even terrestrial power generation are enormous.

References:

1. L. John Perkins, et al. (2004), “On the utility of antiprotons as drivers for inertial confinement fusion”, Nuclear Fusion 44 1097 doi:10.1088/0029-5515/44/10/004

2. Schmidt, G., Gerrish, H., Martin, J. J. (1999), “Antimatter Production for Near-term Propulsion Applications”, 1999 Joint Propulsion Conference

3. Gaidos, G., et al. (1998), “Antiproton-catalyzed microfission/fusion propulsion systems for exploration of the outer solar system and beyond”, 1998. AIP Conference Proceedings, Volume 420, pp. 1365-1372

Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) is poised to be the most ambitious survey of the universe to be conducted in the visible band. It will have a primary mirror spanning a diameter of 8.4 meters, a 9.6 square degrees field of view and a 3.2 Gigapixel camera. This enables the LSST to cover about 10,000 square degrees of sky area in pairs of 15-second exposures twice per night every three nights on average. The rapid cadence of the observing program will generate an unprecedented volume of data. Each night, LSST is expected to generate an average of about 15 Terabytes of raw imaging data. As the survey progresses, the amount of computing power required to process the raw data grows from approximately 100 Teraflops at the start of the survey to 400 Teraflops by the end of the survey.

Credit: LSST Corporation

The LSST Observatory will be situated atop Cerro Pachón in northern Chile (Latitude: S 30° 10′ 20.1′′; Longitude: W 70° 48′ 0.1′′; Elevation: 2123 m). Incident light falling on the telescope’s 8.4 m primary mirror is reflected onto a 3.4 m convex secondary mirror and then reflected again onto a 5 m concave tertiary mirror. The light then enters the camera through three refractive lenses before arriving at the focal plane. The focal plane of LSST’s 3.2 Gigapixel camera is tiled by 189 CCD sensors of 4096 by 4096 pixels each. The CCDs are maintained at an operating temperature of 180 degrees Kelvin and are grouped into 27 rafts of 9 CCDs each.

Credit: Todd Mason, Mason Productions Inc. / LSST Corporation

The 4 main science themes of LSST are:

1. Probing Dark Energy and Dark Matter

2. Taking an Inventory of the Solar System

3. Exploring the Transient Optical Sky

4. Mapping the Milky Way

For objects located within the Solar System, the primary data catalogue that LSST will generate is estimated to contain several million main-belt asteroids, 100,000 Near Earth Objects (NEOs), 100,000 Jovian Trojan asteroids, 40,000 Trans-Neptunian Objects (TNOs) and numerous objects with perihelia at several hundred AU. One AU is the Earth-Sun distance and it has a value of 149.6 million kilometres. Since most of these objects in the Solar System will be observed several hundred times, the orbit of each object can be precisely measured. LSST will be capable of detecting massive objects lurking in the dark outer reaches of the Solar System as it can detect a Pluto-sized object out to a few hundred AU and an Earth-sized object out to over a thousand AU, depending on the albedo of the object.

A nominal operational period of 10 years is anticipated for LSST and it is expected to receive first light in 2015. In terms of 2010 U.S. dollars, the estimate cost for LSST is $455 million for construction and $38 million per year for operations. A key challenge in this project is effective data mining of the unprecedented volume of data that will be generated. The enormous catalogue of data that LSST produces will be made available to the world via the Internet, thereby creating a shared resource for anyone in the world who wants to explore it.

Sailing the Titan Seas

The Titan Mare Explorer (TiME) is a proposed mission to Saturn’s moon Titan which will explore one of the hydrocarbon seas located in the northern region of the moon. TiME will be powered by an advanced Stirling radioisotope generator (ASRG) which provides over 100 watts of electrical power from 500 watts of heat derived from the decay of 800 grams of radioactive plutonium-238. The ASRG has a mass of 28 kilograms and a nominal lifetime of 14 years. This will allow TiME to operate on Titan for a long period of time where solar power is impractical.

TiME is currently in its conceptual phase and if selected, its launch is schedule to take place in 2016, with arrival at Titan in 2023. TiME will make a direct entry into Titan’s atmosphere from its interplanetary trajectory and descend under a parachute before “splashing down” on Ligeia Mare - a hydrocarbon sea in the high northern latitudes of Titan. Ligeia Mare measures roughly 500 kilometres in diameter and it has a surface area of around 100,000 square kilometres. Assuming that all goes well, TiME will perform the first ever exploration of an ocean environment beyond Earth for a nominal duration of 6 Titan days (96 Earth days).

The science objectives of the TiME mission are:

1. Determine the chemistry of a Titan sea.

2. Determine the depth of a Titan sea.

3. Constrain marine processes on Titan.

4. Determine how the local meteorology over the sea varies on diurnal timescales.

5. Characterize the atmosphere above the sea.

Ligeia Mare was first identified from a synthetic aperture radar (SAR) image acquired by the Cassini spacecraft on February 2007. In the SAR image, Ligeia Mare appears to be very dark and this is consistent with a very smooth surface. Radiometry data also indicates a liquid ethane/methane composition for Ligeia Mare. A relatively deep bathymetry is expected for Ligeia Mare since it appears pitch-black in radar observations with no appreciable bottom echo. In comparison, a lake near the South Pole of Titan named Ontario Lacus shows an appreciable bottom echo in radar observations which suggests a depth shallower than 6 meters.

A paper by Ralph D.Lorenz, at al. (2012) entitled “Winds and tides of Ligeia Mare, with application to the drift of the proposed time TiME (Titan Mare Explorer) capsule” investigates how the winds and tides on Ligeia Mare can affect the drift of TiME on the sea’s surface. The proportion of liquid ethane to methane is not known for Ligeia Mare. However, the composition may be ethane-rich since methane evaporates more readily from the sea’s surface. The paper suggests that the composition of Ligeia Mare is expected to have a significant effect on the winds around the sea. A methane-rich composition should result in gentler winds as the evaporating methane absorbs more of the incoming solar energy. On the other hand, an ethane-rich composition should lead to stronger winds. Assuming an ethane-rich sea, the wind speeds at Ligeia Mare appear to rarely exceed 1 m/s and never exceed 1.5 m/s. Furthermore, the likely drift rate calculated for TiME is around one tenth of the wind speed.

Using an assumed bathymetry, the tides in Ligeia Mare is modelled to occur in the form of a slosh between the eastern and western lobes of the sea. The maximum tidal amplitude of approximately 0.6 meters is expected to occur at the far end of the eastern lobe of Ligeia Mare. With the assumed bathymetry, the maximum tidal current of around 1 cm/s can be found in the eastern lobe. This shows that for TiME, its motion on Ligeia Mare is likely to be dominated by wind drag rather than by tidal currents. The drift of a capsule floating on the surface of Ligeia Mare will ultimately depend on the drag forces being exerted by the air and liquid on the cross-sectional areas of the capsule above and below the “waterline” respectively. Nevertheless, the amount of drift and/or drift contributions can be augmented by changing the cross-sectional areas of the capsule above and below the “waterline” or by drag modulation using a small actuator.

Superheated Earth-sized Planets

When a star evolves into a red giant towards the end of its life, planets that orbit the star at sufficiently close distances are expected to be engulfed by the star. Two Earth-mass planets around the hot B subdwarf star KOI 55 could be remnants of the metallic core of a single massive gas-giant planet that spiralled into the stellar envelope of KOI 55 when the star ballooned into a red giant. As the gas-giant planet spiralled in, it released gravitational energy into the red giant which removed most of the star’s outer layers. This process transformed KOI 55 from a red giant star into an extreme horizontal branch (EHB) star, which is essentially the leftover core of the red giant star. Basically, KOI 55 consists of a helium burning core that is surrounded by a thin envelope of hydrogen and it is type of EHB star known as a hot B subdwarf.

The gas-giant planet that spiralled into KOI 55 had all of its gaseous outer layers tidally stripped from it and the leftover metallic core of the planet was probably further destructed into several Earth-mass bodies. For this reason, the two Earth-mass planets around KOI 55 are likely to be two of the surviving Earth-mass bodies. KOI 55 has half the mass of the Sun, one fifth the Sun’s diameter, 23 times the Sun’s luminosity and an effective surface temperature of 27,730 degrees Kelvin. Even though KOI 55 is smaller in size than the Sun, it is many times more luminous because of its very large effective surface temperature. In comparison, the Sun has an effective surface temperature of 5,778 degrees Kelvin.

Credit: Sami Mattila

The two Earth-mass planets around KOI 55 are denoted KOI 55.01 and KOI 55.02 respectively. KOI 55.01 and KOI 55.02 orbit at distances of 0.0060 and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours respectively. 1 AU is a unit of measurement which denotes the mean Earth-Sun separation distance. The orbits of KOI 55.01 and KOI 55.02 are close to a 3:2 resonance where the outer planet orbits twice for every three orbits of the inner planet. Being situated in such close proximity to a hot and luminous star, the dayside temperature of KOI 55.01 is estimated to be 9100 degrees Kelvin while the dayside temperature of KOI 55.02 is estimated to be 8100 degrees Kelvin. This makes the dayside surfaces of both planets hotter than the surfaces of most stars.

	KOI 55.01	KOI 55.02
Orbital Period (hour):	5.7625	8.2293
Orbital Distance (kilometre):	896,980	1,137,490
Orbital Distance (AU):	0.0060	0.0076
Mean Dayside Temperature (Kelvin):	9100	8100
Planet Radius (Earth = 1.0):	0.759	0.867
Planet Mass (Earth = 1.0):	0.440	0.655

KOI 55 is so hot that it emits most of its energy in the form of ultraviolet radiation. Orbiting at such close proximity to KOI 55, the two Earth-mass planets are expected to be blow-torched by intense radiation. This can cause severe ionization and evaporation of the planets’ material, leading to the complete vaporization of both planets within a few tens of million years. However, both planets are likely to posses global magnetic fields that are many time stronger than the Earth’s. A strong magnetic field can substantially reduce the rate of evaporation by keeping the ionized gases from escaping the planet. This can allow both planets to survive through the EHB phase of KOI 55.

The future of these two Earth-mass planets will also depend directly on the evolution of KOI 55. As an EHB star, KOI 55 will eventually run out of helium to burn and settle into a white dwarf star. KOI 55.01 and KOI 55.02 orbit so close to the central star that any increase in the physical size of KOI 55 will destroy the planets prior to the formation of the white dwarf star. However, the size of KOI 55 is not expected to increase and both planets will continue to orbit KOI 55 long after it has turned into a white dwarf star.

References:

1. Charpinet, et al. (2011), “A compact system of small planets around a former red-giant star”, Nature, 480, 496.

2. Ealeal Bear and Noam Soker (2012), “A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616”, arXiv:1202.1168v1

Potentially Habitable Alien World

Gliese 667C c is a potentially habitable planet orbiting one of three stars in a nearby triple-star system. This triple-star system consists of a pair of K-dwarf stars orbiting each other and a fainter red dwarf star orbiting further out, around both the K-dwarf stars. The outer red dwarf star is the star around which Gliese 667C c orbits, and this star is 31 percent as massive and 1.4 percent as luminous as the Sun. Gliese 667C c orbits its host star with a period of 28 days and at a distance that is about 8 times closer than the Earth is from the Sun. Since Gliese 667C c orbits around a faint red dwarf star, the planet receives the same intensity of stellar radiation as the Earth receives from the Sun even though it orbits much closer to its hosts star.

Credit: PHL @ UPR Arecibo/ESO/S. Brunier

The distance of Gliese 667C c from its host star places the planet comfortably within the star’s liquid water habitable zone. This means that temperatures on the surface of Gliese 667C c are expected to be just right to sustain liquid water on the planet’s surface. However, the actual habitability of GJ 667Cc will depend on additional physical parameters such as atmospheric composition, cloud cover and interior dynamics, which are currently unknown. Gliese 667C c is estimated to be at least 4.5 times as massive as the Earth.

As of February 2012, the Planetary Habitability Laboratory at UPR Arecibo regards Gliese 667C c as the fourth potentially habitable planet discovered so far. The other three potentially habitable planets are HD85512 b, Gliese 581 d and Kepler-22 b. The Gliese 667 triple-star system is located at a distance of about 22 light years away towards the constellation of Scorpius. To the unaided eye, this triple-star system appears as a single star with an apparent magnitude of 5.9.

Credit: PHL @ UPR Arecibo

The discovery of Gliese 667C c was first mentioned in a pre-print made public on 21 November 2011 by Xavier Bonfils, et al. entitled “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”. A more recent paper with additional observations of Gliese 667C c was published on 2 February 2012 by Guillem Anglada-Escude, et al. entitled “A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone”.

A Disintegrating Planet

Planets that orbit their host stars with periods of less than one Earth day are entirely possible and a number of these planets have already been discovered. An example is the planet 55 Cancri e which takes just 17.8 hours to orbit around its host star. However, there is considerably less emphasis on the search for such planets because signals that might be indicative of short-period planets tend to be false positives that are associated with the highly diluted light from background binary stars that happen to be located along the line-of-sight to the subject foreground star. Furthermore, giant planets that orbit too close to their host stars can get destroyed when intense stellar radiation causes the planet to puff up and loose material.

A paper by Saul Rappaport, et al. (2012) entitled “Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548” describes the discovery of a possible super-Mercury sized planet that orbits a star slightly cooler than the Sun. Periodic oscillations in the overall brightness of the star KIC 12557548 have been detected by NASA’s Kepler space telescope and these oscillations occur with a periodicity of 15.685 hours. Kepler detects the presence of planets around other stars by looking for small periodic dips in a star’s brightness when a planet crosses in front of its host star. However, the shape of the light-curve representing the dimming of KIC 12557548 is inconsistent with what should be produced by a single transiting planetary body.

KIC 12557548 is a K-type orange dwarf star which is both smaller and cooler than the Sun. With a short period of just 15.685 hours, whatever that is orbiting KIC 12557548 must be in a very close-in orbit. Rappaport et al. suggest that the periodic dimming of KIC 12557548 is caused by a stream of macroscopic dust particles from a disintegrating planet that is larger in size than Mercury. This involves a planet with 10 percent the mass of the Earth (1.8 times the mass of Mercury) and half the diameter of the Earth (1.3 times the diameter of Mercury) orbiting the host star KIC 12557548 at a distance of just 3 stellar radii from the surface of the star. Being so close to its parent star, the planet is expected to be intensely radiated with an estimated peak dayside temperature of 2100 degrees Kelvin. Such a temperature is sufficient to vaporize rock material on the planet’s surface. Since rocks are agglomerations of different individual grains, the vaporized rock material is expected to carry with it a fair amount of still solid grain material.

The solid grains are then carried off the surface of the planet and into space by a thermal Parker-type wind. How such a thermal wind works in when gas drag from the vaporized rock material accelerates the solid grains. At some distance from the planet’s surface, these solid grains are accelerated to speeds exceeding the local escape velocity and escape the planet. An upper limit to the mass loss rate can exist when the outflow becomes too thick, leading to insufficient light from the star reaching the planet’s surface to heat it. Once the solid grains escape the planet and the gas density becomes sufficiently rarefied, the solid grains will decouple from the gas and form a comet-like tail behind the planet. The shape of this comet-like tail is defined by a combination of Coriolis forces and stellar radiation pressure acting on the solid grains. The transit of such a dust tail in front of KIC 12557548 is consistent with the observed dimming characteristics.

In the case for KIC 12557548, a planet that is more massive than the Earth is highly unlikely because the gravity of such a planet will be too strong to allow the dust grains to be accelerated into space. The efficiency of driving dust grains into space via a thermal wind decreases dramatically as the gravity of the planet increases. Future observations can help further confirm the existence of a disintegrating super-Mercury sized planet around the star KIC 12557548. Observations of KIC 12557548 with shorter cadence can look for asymmetries in the ingress and egress portions of the light-curve which might reveal the morphology of the dust tail extending from the disintegrating planet. Additionally, spectral observations using the Hubble Space Telescope can determine if the disintegrating planet is largely made of heavy elements like the Earth and Mercury by searching for absorption line features of metallic elements in the outflow.

Flares from a Supermassive Black Hole

Supermassive black holes are known to exist in the cores of many galaxies. On average, each supermassive black hole in a typical galaxy disrupts a passing star once every 10,000 years or so. An event like this generates a spectacularly bright flare that lasts for months as the disrupted star forms an accretion disc around the supermassive black hole. In the Milky Way galaxy, a supermassive black hole named Sagittarius A* sits in its center. Sagittarius A* is estimated to have over 4 million times the mass of the Sun. On a daily basis, Sagittarius A* is observed to emit tiny flares that lasts for only a few hours each time. These flares are billions of times smaller in amplitude when compared to a flare produced by the disruption of a star.

In a recently published paper by Kastytis Zubovas, et al. 2011, it is postulated that the tiny flares produced by Sagittarius A* on a daily basis are caused by the tidal disruption of asteroids rather than stars. There are vastly more asteroids than stars and asteroids are also much smaller than stars. This explains why the flares produced by Sagittarius A* are much more frequent but are much less luminous than a flare generated by the disruption of a star.

The total luminosity of Sagittarius A* in its quiescent state is approximately 300 times the luminosity of the Sun. This remarkably low luminosity is believed to be powered by a very tenuous quasi-spherical accretion flow of gas into the supermassive black hole. The quiescent state of Sagittarius A* is punctured a few times each day by tiny flares. These flares have luminosities ranging from 3 to 100 times greater that the quiescent state of Sagittarius A* in both X-rays and near infrared.

The minimum size of an asteroid that is necessary to produce an observable flare from Sagittarius A* is estimated to be around 10 kilometres. An asteroid gets tidally disrupted in the vicinity of Sagittarius A* when it passes close enough to the supermassive black hole such that the asteroid’s own gravity becomes unable to hold the asteroid together. This causes the asteroid to break up into smaller fragments that are bound by chemical forces rather than by gravity where the maximum size for such a fragment is probably less than 1 kilometre.

In order for an asteroid to be tidally disrupted, it has to come within 1 AU of Sagittarius A* where 1 AU is basically the average distance of the Earth from the Sun. Sagittarius A* is a supermassive black hole and any object orbiting it within 1 AU will be travelling at an immense velocity of over 60,000 kilometres per second. Since Sagittarius A* is surrounded in its immediate vicinity by a very tenuous gaseous accretion flow, the fragments of a tidally disrupted asteroid will get vaporised by friction with the surrounding gas as they plough through at such incredible speeds. The energy released from the vaporisation of these asteroid fragments is sufficient to produce the observable flares from Sagittarius A*. As the tenuous gaseous accretion flow around Sagittarius A* extends beyond 1 AU, an asteroid passing Sagittarius A* beyond 1 AU and remains intact will still have its surface layers vaporised.

The disruption of a planet around Sagittarius A* is expected to occur much less frequently, on the order of one every thousand years or so. Since a planet is much more massive than an asteroid, a flare produced from the vaporised fragments of a tidally disrupted planet is expected to be millions of times more luminous than from an asteroid. In fact, an observed X-ray echo from a giant molecular cloud that is located a few hundred light years away from Sagittarius A* points towards the possibility that a large flare may have been produced from the disruption and subsequent vaporisation of a planet, occurring approximately 300 years ago. 

A Pulsar’s Diamond Planet

A pulsar is a rapidly spinning and strongly magnetized neutron star which emits an intense bipolar beam of electromagnetic radiation that generally does not coincide with the spin axis of the pulsar. This results in the ‘lighthouse effect’ when the beam of electromagnetic radiation happens to be orientated towards the Earth; leading to an apparent pulsed nature as the beam of emission periodically sweeps past the Earth. Hence, the pulsar derives its namesake from this observable behaviour. For some pulsars, the periodicity of its pulsed nature can be as precise as an atomic clock. A pulsar forms out from the ultra-compressed core of a massive star during a supernova explosion and a typical pulsar contains an entire Sun’s worth mass of matter packed into an incredibly small volume with a diameter of no more than a few tens of kilometres. On average, each cubic centimetre of a pulsar’s material holds on the order of a few hundred billion metric tons of mass.

PSR J1719-1438 is a pulsar with a spin period of 5.7 milliseconds, which means that it spins 175 times each second. The very rapid spin rate of PSR J1719-1438 means that it is categorized under a unique group of pulsars called millisecond pulsars. The high spin rate of a millisecond pulsar is believed to be cause by the spinning-up of a pulsar by the accretion of matter from a binary companion which transfers angular momentum to the pulsar. A recent paper by M. Bailes et al that is titled “Transformation of a Star into a Planet in a Millisecond Pulsar Binary” describes the discovery of a Jupiter-mass companion in a 2.2 hour orbit around the pulsar PSR J1719-1438. The paper also investigates the possibility that PSR J1719-1438 was once an ultra compact low-mass X-ray binary (UC LMXB), whereby matter was accreted by the pulsar from a binary companion star. Almost all of the material from the binary companion star was accreted by the pulsar, leaving behind a Jupiter-mass remnant of what was once the companion star.

The Jupiter-mass companion orbiting around PSR J1719-1438 was detected from slight pulse-timing variations observed in the pulsar’s extremely regular pulses. These pulse-timing variations are caused by the gravitational tugging of the pulsar by its Jupiter-mass companion. The Jupiter-mass companion is orbiting so close to PSR J1719-1438 that it cannot be anywhere as large in size as Jupiter because at that size, its own gravity will not be sufficiently strong enough to prevent it from being gravitationally torn apart by the nearby pulsar. For this reason, the Jupiter-mass companion must be much more compact in nature to avoid being gravitationally torn apart. A lower limit of 23 grams per cubic centimetre is required for the mean density of the Jupiter-mass companion such that its overall physical size is compact enough for its own gravity to be sufficiently strong to prevent it from being gravitationally torn apart. In comparison, the mean density of Jupiter is less than 2 grams per cubic centimetre.

Such a high density means that the Jupiter-mass companion orbiting around PSR J1719-1438 cannot be a gas-giant planet like Jupiter as it is too dense to be made up of just hydrogen and helium like Jupiter. Instead, the Jupiter-mass companion is theorized to be what remains of the degenerate core of the companion star whose material was stripped away and accreted by the pulsar. In this scenario, the Jupiter-mass companion was once a white dwarf star in a tight orbit around PSR J1719-1438. The white dwarf star was basically what remained of a star like our Sun after it had extinguished its hydrogen and helium in its core through fusion of these elements into carbon. So, if the Jupiter-mass companion is what remains of the core of the white dwarf star, it is expected be comprised of heavier elements such as carbon. This will allow it to have a mean density of 23 grams per cubic centimetre or more, making it compact enough such that it will not be gravitationally torn apart by the nearby pulsar.

The Jupiter-mass companion around PSR J1719-1438 is a very unique case because most white dwarfs stars tend not to survive the transfer of matter to their companion pulsars. A typical white dwarf star tends to be too close to its companion pulsar by the time it starts transferring material to the pulsar. This leads to the complete destruction of the white dwarf star, possibly leaving behind a disk of remnant material orbiting around the pulsar. Such a disk of material is expected to coalescence into a planetary system of Earth-mass planets rather than a Jupiter-mass object. Therefore, a Jupiter-mass companion around a pulsar will require a low mass white dwarf star so that the transfer of matter to its companion pulsar occurs at a further distance away. This can allow the transfer of matter to cease just in time before the complete destruction of the white dwarf star, leaving behind the remnant core as a Jupiter-mass object.

Of all the pulsars discovered so far, only a handful of them have planetary-mass companions. This means that the formation of planetary-mass companions around pulsars is the exception rather than the rule. Furthermore, the companion around PSR J1719-1438 is the only known Jupiter-mass object around a pulsar. All previously discovered planetary-mass objects around pulsars are around the mass of the Earth. The case for PSR J1719-1438 requires a very unusual combination of white dwarf mass and composition. This unique combination allows PSR J1719-1438 to transform its companion into very rare and exotic type of planet. The Jupiter-mass companion around PSR J1719-1438 is likely to be entirely composed of crystallized carbon, which is also known on Earth as diamond.

Land Planets and Ocean Planets

The region around a star where an Earth-like planet can maintain liquid water on its surface is known as the habitable zone or the ‘Goldilocks’ zone. Previous studies of Earth-like planets in the habitable zones of stars generally assume ocean covered planets that resemble the present Earth. If such an ocean planet is too far from its star, it leads to an ice-albedo feedback which ends in the complete freezing of the planet. If the same planet is too close to its star, a runaway greenhouse effect occurs which ends in the complete evaporation of the planet’s oceans. Now imagine another kind of habitable planet whose surface is predominantly land, with only small areas of surface water. A planet like this is can be called a land planet.

Although a land planet is probably covered by vast deserts, it can support localized regions with abundant water and such regions can exist for example, near the poles of the planet. In our own solar system, the closest analogy to a land planet is Saturn’s moon Titan. Titan has lakes of methane on both its poles and between the poles of Titan is a vast desert that spans the tropics and temperate zones. The surface of Titan is far too cold for liquid to exist, resulting in liquid methane playing its role on Titan as water does on Earth. A rather engaging paper by Abe et al. 2011 that is entitled “Habitable Zone Limits for Dry Planets” studies the possibilities that land planets can have wider habitable zones than ocean planets. This means that a land planet can be nearer or further from its parent star than an ocean planet and still be capable of supporting habitable Earth-like surface conditions.

In this article, the comparison between an ocean planet and a land planet assumes that each planet orbits a Sun-like star that is identical to ours. The complete freezing of an Earth-like ocean planet occurs when the Sun is dimmed to 90 percent of its present luminosity while the complete freezing of a land planet only occurs when the Sun is dimmed to 77 percent of its present luminosity. In other words, a land planet has a greater resistance to complete freezing than an ocean planet. This is due to the fact that a land planet will tend to be less reflective than an ocean planet. One reason for this is that a land planet has fewer clouds than an ocean planet because it is less humid. The other reason is that less snow accumulates on a land planet than on an ocean planet because the atmosphere is drier and the daytime temperatures are higher for a land planet. Fewer clouds and less snow cover make a land planet less reflective than an ocean planet to incoming insolation from its parent star. A less reflective planet means a higher surface temperature. For this reason, a land planet can be further than an ocean planet from its parent star before complete freezing occurs. Therefore, the outer boundary of the habitable zone of a land planet is larger than for an ocean planet.

Moving now to the inner boundary of the habitable zone, liquid water can remain stable on an ocean planet until the Sun is brightened to 135 percent or more of its present luminosity. For a land planet, liquid water can remain stable on its surface until the Sun is brightened to 170 percent or more of its present luminosity. This means that the inner boundary of the habitable zone of a land planet is closer in to its parent star than for an ocean planet since a land planet can be nearer to its parent star than an ocean planet before a runaway greenhouse effect occurs. For an ocean planet, a runaway greenhouse effect occurs when there is enough water vapour in the atmosphere such that the atmosphere becomes optically thick to outgoing thermal radiation. This causes the ocean planet to absorb more energy from its parent star than it can radiate away, eventually causing the surface of the planet to become sterilizingly hot.

Compared to an ocean planet, the case for a land planet is rather different. The low latitude region of a land planet is expected to have an extremely low humidity and effectively no surface water. This allows the low latitude region of the land planet to absorb more energy from its parent star than it can radiate away, without leading to a runaway greenhouse effect. Furthermore, for an extremely dry land planet, all its surface water can evaporate without a significant contribution of water vapour into the atmosphere to trigger a runaway greenhouse effect. Since a runaway greenhouse effect may not occur for a land planet, the equivalent runaway greenhouse effect threshold can be defined as the maximum insolation the land planet can receive, beyond which all surface water and surface ice completely evaporate, including even those at the poles.

A land planet with no permanent surface water can still sustain a hydrated layer of surface soil by the deposition and subsequent melting of frost. At night, it may be cold enough for frost to form, especially within the pore spaces of the surface soil. During the day, the frost can melt into liquid water and moisturize the surface soil. This mechanism is particularly effective for a land planet with a thin atmosphere since a thin atmosphere is much less effective at damping daily temperature fluctuations than a thick atmosphere. Nights on a land planet with a thin atmosphere can get exceptionally cold, thereby creating an environment that is very conducive for the formation of frost. Additionally, a thinner atmosphere will reduce the rate of energy transport from the equator to the poles of a land planet. This stabilizes any polar ice caps against evaporation and reduces the input of water vapour into the planet’s atmosphere which further prevents the onset of a runaway greenhouse effect.

To conclude, the habitable zone for a land planet around its parent star is considerably larger than it is for an ocean planet around the same star. One key consideration that can be of importance is that the presence of clouds creates a major uncertainty as to the true limits of the habitable zone of a planet around its parent star. Clouds can warm or cool a planet, whereby high clouds have a warming effect and low clouds have a cooling effect. Reducing the coverage of high clouds and increasing the coverage of low clouds pushes the inner limit of a planet’s habitable zone closer to its parent star. Alternatively, increasing the coverage of high clouds and reducing the coverage of low clouds pushes the outer limit of a planet’s habitable zone further from its parent star.

The Sun’s luminosity increases at a rate of about 9 percent per billion years. As the Sun brightens, it might be possible that an ocean planet like the Earth can lose most of its water and become a land planet without passing through a sterilizing runaway greenhouse effect. However, this depends on how much water an ocean planet like the Earth can lose before it reaches the threshold for a sterilizing runaway greenhouse effect. Still, even if an ocean planet can successfully evolve into a land planet, the surface temperature of the planet during the transition phase can reach up to between 300 to 400 degrees Kelvin. Such conditions are marginally habitable as only thermophilic microbial life on Earth can exploit such conditions. Nevertheless, the possibility of the Earth becoming a land planet in the far future adds an extra billion years or so to the continuous habitability of the Earth even as the Sun evolves to a higher luminosity.

Tight Stellar Binary

The discovery of a detached pair of white dwarfs with a 12.75 minutes orbital period has been published by Warren R. Brown et al. 2011 in a paper entitled: “A 12 minute Orbital Period Detached White Dwarf Eclipsing Binary”. This stellar system is designated SDSS J065133.33+284423.3 or just J0651, and it is the tightest white dwarf binary system yet discovered. J0651 is located at a distance of over 3000 light years from the Sun. Both white dwarfs are racing around each other at over 600 kilometers per second. The visible primary is a 0.25 solar mass tidally distorted helium white dwarf while the unseen secondary is a 0.55 solar mass carbon-oxygen white dwarf.

Credit: David A. Aguilar (CfA)

Both white dwarfs are separated by a mean distance of less than one-third the separation between our Earth and the Moon, and they are on the brink of a merger. The two white dwarfs are expected to merge in 900 thousand years from the loss of energy and angular momentum via the emission of gravitational wave radiation. This will eventually lead to a massive rapidly spinning white dwarf, the formation of a stable interacting binary, or possibly an explosion as an underluminous type Ia supernova. The orientation of the orbits of both white dwarfs in the binary system is such that eclipses of each white dwarf by the other are observable and this allows accurate measurements of the orbital parameters, masses and radii of the white dwarfs.

The eclipse of one white dwarf by the other occurs like clockwork, at a very predictable rate. Observers on a hypothetical planet which orbits around this star system will see one of their two suns disappear every 6 minutes or so. The shrinking of the orbits of both white dwarfs via the emission of gravitational wave radiation is expected to be measurable from observing changes in the eclipse timings. This provides a remarkable opportunity to test for the existence of gravitational waves that are predicted by Einstein’s general theory of relativity.