The Runaway Giant

The Large Magellanic Cloud is a nearby irregular galaxy that is located about 160 thousand light years away and it is also a satellite galaxy of the Milky Way. Extremely massive stars will up to 300 times the mass of our Sun are known to exist in a massive star cluster called R136 which is located near the center of the Tarantula Nebula, in the Large Magellanic Cloud. Residing in R136 is a star called R136a1 and this star is currently on record as the most massive star known, with a colossal mass that is estimated to be 265 times the mass of our Sun. Just after birth, R136a1 is estimated to have 320 times the mass of our Sun, having lost 50 solar masses over the past million years! R136a1 also hold the record for the most luminous star known as it blazes with 10 million times the luminosity of our Sun.

Located at a projected distance of 95 light years from the massive star cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud is a very massive star called VFTS 682. Spectroscopic observations have revealed VFTS 682 to be a hydrogen-rich Wolf-Rayet star. Wolf-Rayet stars are massive stars which lose mass rapidly by emitting very strong stellar winds at speed of up to a couple of thousand kilometers per second. What makes VFTS 682 perplexing is that this star is one of the most massive stars found in isolation. Very massive stars generally reside in the centers of massive star clusters since the formation of such objects are generally known to occur in the dense environments found in the centers of massive star clusters. The presence of such an extremely massive star outside the massive star cluster R136 presents the question of whether it was ejected from R136 or did it form in isolation instead.

The physical properties of VFTS 682 are impressive as VFTS 682 is estimated to have over 3 million times the luminosity of our Sun and a mass on the order of 150 times the mass of our Sun. VFTS 682 is a single isolated star as it shows no signs of binarity. Spectroscopic observations have shown that in terms of spectral appearance, VFTS 682 is almost identical to another very massive star called R136a3 which is located in the core of the massive star cluster R136. From velocity measurements, VFTS 682 is estimated to have a true velocity of 40 kilometers per second with respect to R136, placing it in the lower range of velocities for runaway stars. If VFTS 682 is indeed a runaway star, it will be the most massive one known to date and a bow shock might even be observable around VFTS 682 as it is surrounded by dust clouds.

Very massive stars are know to form in dense cluster environments where they are generally found because the very short lifespans of very massive stars mean that they have insufficient time to travel far from where they were born. VFTS 682 is indeed a very massive star in isolation and this creates an interesting challenge for dynamical ejection scenarios and massive star formation theory. The paper detailing this discovery is by Joachim M. Bestenlehner et al and it is entitled “The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136”.

Lava-Ocean Planets

CoRoT-7b is the first characterised rocky super-Earth exoplanet and it orbits extremely close to its parent star, at a distance of only 2.56 million kilometres which translates to just 4.48 stellar radii of its parent star. CoRoT-7b is located so close to its parent star that the length of one year on this planet is a fleeting 20 hours and 29 minutes. The spin and orbit of CoRoT-7b are likely synchronized, resulting in a hemisphere of continuous daylight and a hemisphere of continuous night. CoRoT-7b is measured to have 1.58 times the diameter and 6.9 times the mass of the Earth.

CoRoT-7b is not expected to have any appreciable atmosphere as the scorching environment on the planet does not support the presence of significant amounts of volatiles that can make up an atmosphere. Any atmosphere on CoRoT-7b is expected to be extremely rarefied. Hence, the transport of heat by any planetary scale winds on CoRoT-7b will be unable to significantly change the temperature distribution on the dayside or provide heat to the nightside, leading to very low surface temperatures on the nightside of the planet. This enables a huge surface temperature difference between the dayside and nightside of the planet to be maintained.

At the sub-stellar point on the dayside hemisphere of CoRoT-7b, the estimated temperature is a roasting 2470 degrees Kelvin. The sub-stellar point on the surface of CoRoT-7b has a zenith angle of zero and on this spot the host star of CoRot-7b is always directly overhead, making the sub-stellar point the hottest spot on the surface of the planet. An ocean of molten rocks is believed to be present on the extremely hot star-facing hemisphere of CoRoT-7b. High temperatures of well over 2000 degrees Kelvin on most of the dayside hemisphere of CoRoT-7b mean that the viscosity of the molten rocks that make up the lava ocean is probably much closer to that of water that to that of Earth’s lavas.

In order to compute the extent of coverage of the lava ocean on CoRoT-7b, certain assumptions have to be made. If Coriolis forces are negligible, such a lava ocean will have radial symmetry around the sub-stellar point which enables its extent to be characterized solely by the zenith angle of the ocean’s shore from the sub-stellar point. The ocean’s shore is basically the location on the planet’s surface where the solidification of molten rocks begins to occur.

If the circulation within the lava ocean is extremely efficient in transporting heat, it could lead to an ocean with a uniform temperature. Assuming that the lowest possible temperature of such a lava ocean is 2150 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 75 degrees from the sub-stellar point. This corresponds to 37 percent of the planet’s surface area being covered by the lava ocean. This estimate of the ocean’s size is probably a maximum and it can be seen that lava ocean is limited to just the dayside of CoRoT-7b. This means that circulation within the lava ocean cannot carry any heat from the dayside to the nightside of the planet.

If heat transport within the lava ocean via circulation is not present, then the physical extent of the lava ocean on CoRoT-7b will be smaller. In this case, assuming that the solidification of molten rocks begins to occur at 2200 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 52 degrees from the sub-stellar point. This corresponds to 19 percent of the planet’s surface area being covered by the lava ocean.

Along the shores of the lava ocean, crystallization and condensation of molten rock can occur to create pieces of rocks that sink back to the ocean floor. Also, along the shores of the lava ocean, condensation of molten rock material onto the continental edges can cause the loaded continental edges to progressively sink as it base dissolves into the mantle of the planet. The transport of silicates from the melted base of the continental edges back to the ocean floor can close the circulation of materials. Compared to the Earth’s oceans, any form of wind driven waves on the lava ocean of CoRoT-7b will be very small due to the extremely rarefied atmosphere, the higher viscosity of lava as compared to water and the higher surface gravity of CoRoT-7b as compared to the Earth.

The nightside of CoRoT-7b will be extremely cold due to the lack of any form of mechanism that can efficiently transport heat from the dayside to the nightside of the planet. The only form of heating on the nightside of CoRoT-7b will be geothermal heating from the decay of radioisotopes within the planet. This leads to a surface temperature of between 50 to 75 degrees Kelvin on the frigid nightside of CoRoT-7b. The paper detailing this study is by Alain Leger et al (2011) and it is entitled “The extreme physical properties of the CoRoT-7b super-Earth”.

The existence of a lava ocean on CoRoT-7b should also be common to many small and very hot rocky planets that orbit extremely close to their host stars. A recently discovered planet called Kepler-10b has a lot of resemblance with CoRoT-7b, but its properties are expected to be even more extreme as it has a higher temperature at its sub-stellar point and possibly a larger lava ocean. To conclude, a new class of planets termed “lava-ocean planets” may be prevalent amongst small and very hot rocky worlds with ‘star-hugging’ orbits.

Ultra-Hot Super-Earth

55 Cancri is a yellow dwarf star that is located just 41 light years away from Earth in the direction of the constellation of Cancer. This star has a slightly lower mass and a slightly lower luminosity as compared to our Sun. As of 2010, five extrasolar planets are known to orbit 55 Cancri. The innermost planet is a terrestrial super-Earth planet with a few times the mass of our Earth while the outer 4 planets are gas giant planets with masses similar to Jupiter. A recent paper by Winn et al. (2011) that is entitled “A Super-Earth Transiting a Naked-Eye Star” describes the detection of transits of the innermost planet which orbits 55 Cancri. The innermost planet is designated 55 Cancri e and it was previously discovered in 2004 from radial velocity measurements.

55 Cancri e was formerly reported to have an orbital period of 2.808 days, but this value has since been revised down to just 0.7365 days or 17 hours and 41 minutes. “You could set dates on this world by your wristwatch, not a calendar,” study co-author Jaymie Matthews of the University of British Columbia said in a statement. This revision to the planet’s orbital period increased the likelihood that the planet could transit its host star from an initial probability of 13 percent to 33 percent. Observations by the Microvariability and Oscillations of STars telescope (MOST) lead to the discovery of the transits of 55 Cancri e in front of its host star. Each transit of 55 Cancri e lasts just over 100 minutes in duration and during each transit, 55 Cancri e blocks just 0.018 percent of the light from its host star.

From the amount of dimming imposed by the transit of 55 Cancri e in front of its host star, the diameter of 55 Cancri e is estimated to be 20800 kilometres, making this planet 63 percent larger than the Earth in diameter. Radial velocity measurements have also shown that 55 Cancri e has 8.57 times the mass of the Earth. With the size and mass of the planet known, the mean volumetric density of 55 Cancri e is estimated to be 10.9 grams per cubic centimetre, making this planet twice as dense as the Earth and the densest solid planet found anywhere so far. This suggests a rock-iron composition that is similar to the Earth under significantly more gravitational compression.

The amazingly short orbital period of 55 Cancri e means that this planet is located only 1.5 million kilometres from the fiery surface of its host star. In this extreme infernal environment, the temperature at the substellar point of 55 Cancri e could approach 3000 degrees Kelvin if the planet is tidally locked and if the incoming heat remains on the dayside. However, if the heat is distributed over the entire surface of the planet and if the planet has an albedo of zero, the temperature will be a lower but still blistering 2100 degrees Kelvin.

It is unlikely that 55 Cancri e can hold on to an atmosphere that is comprised of gases with low molecular weights. However, volcanic activity on 55 Cancri e can sustain a thin atmosphere with gases of high molecular weights. The presence of an atmospheric wind on 55 Cancri e could shift the hot spot away from the planet’s substellar point. On the surface of 55 Cancri e, any object will weigh 3 times heavier than it does on Earth. During the day, the host star of 55 Cancri e will appear thousands of times brighter and tens of times larger than our Sun appears from the Earth.