Observing the Elias 2-24 Protoplanetary Disk with the Atacama Large Millimeter/Submillimeter Array.

The Ophiuchus Molecular Cloud is a dense molecular cloud roughly 125 parsecs (408 light years) from Earth in the constellation of Ophiuchus, that forms one of the closest areas of star-formation to the Earth. This region contains over 200 known T Tauri stars (very young stars which have not yet begun to generate heat by hydrogen fusion, but which produce considerable energy through gravitational heating) and at least 16 protostars (stars which are still gaining mass by accretion from a surrounding disk, the accretion disk, and are emitting ionised material in jets from their poles). Elias 2-24 is a T Tauri star within the Ophiuchus Molecular Cloud with an estimated age of 400 000 years and mass roughly equal to that of the Sun. This star is surrounded by a protoplanetary disk (a dense structure from which planets are thought to form) from which matter is still actively accreting onto the star, and which is roughly edge on when seen from Earth, making it a good candidate for observation by astronomers trying to understand these structures.

In a paper published on the arXiv database at Cornell University Library on 18 November 2017, and accepted for publication in the Astrophysical Journal Letters, a team of scientists led by Lucas Cieza of the Facultad de Ingenier a y Ciencias, N ucleo de Astronom a at the Universidad Diego Portales, and the Millennium Nucleus Center of Protoplanetary Disks in ALMA Early Science, describe the results of a study of the Elias 2-24 Protoplanetary Disk made with the Atacama Large Millimeter/Submillimeter Array (ALMA) on 13 and 14 July 2017.

Cieza et al. immaged the Elias 2-24 Protoplanetary Disk at a range of wavelengths, intended to detect the densities of different molecules. Molecules will absorb light as energy across a broad part of the spectrum, but can only absorb a finite amount of light before being forced to re-emit some of this energy. However this energy is not released in random bursts, but radiated at specific frequencies determined by the atoms present in the molecule, which atoms are bound to which other atoms, and even which isotopes of each element are present. This gives each molecule its own unique spectrographic signature, which can be used by astronomers to detect different molecules in distant objects such as protoplanetary disks.

Using this method Cieza et al. were able to detect three distinct gaps in the protoplanetary disk around Elias 2-24, at distances of 20, 52, and 87 AU from the star (i.e. 20, 52 and 87 times as far from the star as Earth is from the Sun), and have widths of 6, 28 and 11 AU, respectively. Such gaps in protoplanetaty disks are thought to be caused by the formation of planets, as matter from the disk accretes onto the forming protoplanetary body. Calculating the amount of matter that would be missing from the disks in this gap, Cieza et al. suggest that enough material has been used to form planets with masses of 4, 20 and 10 times that of Jupiter, though they do not believe that all of the missing material would have been used up by planetary formation; much of it is likely to have been ejected from these gaps by tidal forces generated by the forming protoplanets.

 Composite ALMA image of the Elias 2-24 Protoplanetary Disk, assembled from averaged images at different wavelengths. Cieza et al. (2017).

Cieza et al. further note that the temperatures at the inner two gaps, 23 and 15 K corresponds closely to those predicted for the snow-lines of Carbon Monoxide (23-28 K) and Nitrogen (12-15 K), i.e. the temperatures at which these molecules with cease to be disassociated gases and start to accrete into snow, suggesting that such snow-formation plays a role in the early stages of planetary formation.

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Faint companions discovered to two planet-hosting stars.

Almost all planets orbiting other stars have been discovered by one of two planetary detection techniques: occlusion, on which the planet passes in front of the star, causing the amount of light reaching us from that star to dim ever so slightly on a regular timescale, or radial velocity, in which the gravity of the planet causes the host star to wobble back and forth by a small, but detectable, amount, again on a regular timescale. However both of these techniques can be hampered if stars thought to be singular are in fact part of a binary system, in which a smaller stellar companion causes the main star to wobble instead of or in addition too the wobble caused by any planet, and the occlusion of one star by another causes brightening and dimming just as well as any planet. Since it is thought that about half the stars in our galaxy are part of binary systems, this is a serious consideration for planet-hunters.

In a paper published on the online arXiv database at Cornell University Library on 2 September 2016, Justin Wittrock and Stephen Kane of the Department of Physics & Astronomy at San Francisco State University, Elliot Horch of the Department of Physics at Southern Connecticut State University, Lea Hirsch of the Astronomy Department at the University of California at Berkeley, Steve Howell of the NASA Ames Research Center, David Ciardi of the NASA Exoplanet Science Institute, Mark Everett of the National Optical Astronomy Observatory, and Johanna Teske of the Carnegie Department of Terrestrial Magnetism, describe the discovery of two faint companions to planet hosting stars made with the Differential Speckle Survey Instrument at the Gemini-North Observatory.

The first system examined is the HD 2638 system, which currently comprises a G5V star (Yellow Dwarf star slightly smaller than our Sun), 49.93 parsecs (162.8 light years) from us in the constellation of Cetus. This star is calculated to be about 5.1 billion years old, and to have about 87% of the Sun's mass and an effective surface temperature of 5173 K (compared to 5778 K for our Sun). This star is thought to be orbited every 3.43 days by a planet with a mass 48% of that of Jupiter, orbiting at a distance of 0.044 AU (i.e. 0.044 times the distance between the Earth and the Sum or about 10% of the distance at which Mercury orbits the Sun.

A companion star to this system was announced by another team in 2015 (between the time when the Gemini-North observations were made and the publication of the results), a discovery which Wittrock et al. confirm. By their calculations this star is an M1V (Red Dwarf) star with a mass 48% of that of the Sun and an effective surface temperature of 3571 K, orbiting the main star at a distance of 25.5 AU (for comparison Uranus orbits the Sun at a distance of 19.2 AU).

Gemini DSSI speckle image of HD 2638 at 880 nm. The field-of-view is 2.8 × 2.8”. North is up and East is to the left. The source in the center is HD 2638, and a bright source to the bottom and slightly to the left of the main star is the stellar companion. Wittrock et al. (2016).

Potentially the presence of a second star in a stellar system can influence both the possibility of planet forming and our ability to detect them. Wittrock et al. calculate that the presence of a second, smaller star orbiting the main star in the HD 2638 system at a distance of 25.5 AU would prevent any planets from forming and remaining in stable orbits outside 8 AU from the main star - not an issue for the known planet of HD 2638, which orbits at a distance of 0.044 AU. The planet was detected by the radial velocity technique; it was possible to detect the wobbling of the main star caused by a planet orbiting it every 3.4 days. Potentially this signal could be influenced by the gravity of a second star, but Wittrock et al. calculate that the second star would have an orbital period of 110 years, so confusion of the signal caused by that of the planet and that of the star seems highly unlikely.

The second system discussed is HD 164509, another G5V (Yellow Dwarf) star, 52.44 parsecs (171 light years) from Earth in the constellation of Ophiuchus. The primary star in this system is calculated to be 3.2 billion years old and to have 1.10 times the mass of the Sun and an effective surface temperature of 5860 K. This is orbited every 282 days by a planet with a mass 48% of that of Jupiter at a distance of 0.875 AU (for comparison Venus orbits the Sun at 0.723 AU ans Earth at 1 AU).

Wittrock et al. announce the discovery of a previously unknown companion to this star, a Red Dwarf star with a mass 42% of the Sun's and an effetive surface temperature of 3446 K, orbiting at a distance of 36.5 AU (for comparison Neptune orbits the Sun at a distance of about 30 AU, and Pluto at an average distance of 39.5 AU.

Gemini DSSI speckle image of HD 164509 at 880 nm. The field-of-view is 2.8×2.8”. North is up and East is to the left. The source in the center is HD 164509, and a bright source to the bottom and right of the main star is the stellar companion. Wittrock et al. (2016).

Again Wittrock et al. note that such a planet could affect the formation, orbital stability and detectability of any planets in the system; the presence of the second star would prevent planets from forming in a stable orbit more than 13 AU from the primary star - again not an issue for the close-in planet of HD 154509. This planet was also detected by the radial velocity method, though in this case it would take the secondary star 180 years to complete a single orbit about the primary star, so it is unlikely that this star could be responsible for the signal interpreted as coming from the planet.

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When Schotz’s Star passed through the Solar System’s Oort Cloud.                                                It is estimated that about 30% of impact craters larger than 10 km on the Earth and the Moon have been caused by long period comets originating in the Oort Cloud (the very outermost part of the Solar System, between about 30 AU and about 120 000 AU from the Sun, which is to say between 30 and 120 000 times as far from the Sun as the Earth. In the 1980s it...

The planets of NN Serpentis.                       The NN Serpentis system comprises a young binary pair of stars 1670 light years from Earth in the constellation of Serpens. The pair comprises a White Dwarf star with 53.5% of the Sun's mass and a Red Dwarf star with 11% of the Sun's mass orbiting at a distance of 1.5 AU (1.5 times the distance at which the Earth orbit's the Sun, or slightly under the radius of...

Citizen scientists discover a sub-Jovian planet in a quaternary star system.               The Planet Hunters is a citizen science project (i.e. a science project carried out largely by members of the public) which asks visitors to a website to look at randomly selected 30 light-curves from the Kepler Space Telescope, and look for patterns. Volunteers who find patterns that may indicate planets can...

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Three giant exoplanets in very wide orbits around young stars.

In the past two decades a large number of planets have been discovered orbiting other stars (exoplanets). The vast majority of these have been large planets orbiting close to their host stars, such planets being easier to detect due to the influence that their gravity has on the star. Planets further from their stars are harder to detect, as their gravity has less effect upon the star, and they have long orbital periods which will tend to mask this anyway. Such planets are more likely to be detected by direct imaging, though this will require separate observations over a long period of time to confirm the relationship with the host star.

In a paper published on the online arXiv database at Cornell University Library on 29 November 2013 and accepted for publication in The Astrophysics Journal, a team of scientists led by Adam Kraus of the Department of Astronomy at the The University of Texas at Austin and the Harvard-Smithsonian Center for Astrophysics describe three giant planets orbiting young stars at distances in excess of 100 AU (i.e. more than 100 times the distance at which the planet Earth orbits the Sun). All three bodies had previously been noted as potential planets over a decade ago, but can only now be confirmed as objects in orbit about their stars, due to follow-up observations that have tracked their movement. 

Large planets in very wide orbits about young stars presents a considerable challenge for conventional models of planet formation, since it should in theory take far longer for a planet to form this far from a star, potentially never accreting into a large body at all. Our own system contains considerable material beyond the orbit of Neptune (30 AU from the Sun), but this has apparently never accreted into a large planet, despite the 5 billion year age of the Solar System.

The first of the three new planets orbits the binary system FW Tau AB near the center of the Taurus- Auriga complex, 473 light years from Earth. The FW Tau system comprises two red dwarf stars  (FW Tau A and FW Tau B) each though to have a mass 28% of our Sun's orbiting one another at a distance of 11 AU (11 times the distance between the Sun and the Earth). The system is thought to be about 1.8 million years old. The planet, FW Tau b (when naming objects in other stellar systems stars are given upper case letters and planets lower case letters), is calculate to have a mass 10 times that of Jupiter and orbit at a distance of 330 AU (over 10 times the distance at which Neptune orbits the Sun). 

Infrared image of the FW Tau system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The second new planet orbits the binary star ROXs 42B, 440 light years from Earth in the constellation of Ophiuchus. The binary system comprises two stars with masses of 89% that of the Sun and 36% that of the Sun, orbiting at a distance of less than 10 AU. The system is thought to be 6.8 million years old. The new planet, ROXs 42B b is calculated to orbit this pair at a distance of 140 AU and have a mass 10 times that of Jupiter. A second, potential body in the system, provisionally dubbed ROXs 42B cc1, is thought to be a background star.

Infrared image of the ROXs 42B system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The third new planet orbits ROXs 12, another star in the constellation of Ophiuchus, 391 light years from Earth. ROXs 12 is thought to have a mass 87% of that of the Sun and to be 7.6 million years old. ROXs 12b orbits this star at a distance of 210 AU, and has a mass 16 times that of Jupiter.

Infrared image of the ROXs 12 system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

See also Kepler 63b; a giant planet in a polar orbit, A superjovian exoplanet directly imaged orbiting the A-class pre main-sequence star HD 95086, A fourth body in the KOI-13 system, Wasp-12b; slowly boiling away... and KOI-13b, a big, hot planet not a Brown Dwarf.

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The atmosphere of GJ 1214b.

GJ 1214b is a 'Super-Earth' planet (a planet significantly larger than Earth but significantly smaller than Uranus or Neptune) orbiting a Red Dwarf star in the constellation of Ophiuchus, 42 light years from Earth, the discovery of which was announced in a paper in the journal Nature by a team of scientists led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. It orbits the star at a distance of 0.0143 AU (1.43% of the distance at which the Earth orbits the Sun) with an orbital period of 38 hours. 

Since it passes in front of the star when seen from Earth, it is was possible for the discovering team to determine not just the mass of the planet (by the amount it caused the star to wobble) but also its diameter, enabling the mean density of the planet to be determined; 1870 kg mˉ³, compared to 5150 to kg mˉ³ for the Earth. Since a planet significantly larger than the Earth with a similar composition would be expected to be much denser (due to the higher gravity) it was clear that GJ 1214b was a new class of object. Based upon this the discoverers concluded that GJ 1214b was a water world; a small rocky world covered by an ocean hundreds of kilometers deep, with a thin hydrogen/helium atmosphere (water could exist on a planet this close to the star GJ 1214 because it is a lot cooler than our Sun).

Model of GJ 1214b as a waterworld. John Garrett.

However this is not the only possible interpretation of the density of GJ 1214b; a small, dense, rocky planet with an extensive atmosphere could have a similar overall density.

In a paper published on the online arXiv database at Cornell University Library on 23 May 2013, a team of scientists led by Ernst de Mooij of the Department of Astronomy and Astrophysics at the University of Toronto, describe the results of a spectographic study of the atmosphere using the Very Large Telescope in the Atacama Desert in Chile, the William Hershel Telescope on La Palma in the Canary Islands and the Isaac Newton Telescope at Herstmonceux in Sussex, England.

De Mooij et al. predicted the intensity of light that would be produced by the scattering of light from GJ 1214 through the atmosphere of GJ 1214b if the planet had a thin atmosphere with abundant water, a thick atmosphere dominated by hydrogen or a thick, cloudy, hydrogen dominated atmosphere and then plotted the intensity of light detected by these, and previous, observations against this.

Transmission spectrum of GJ1214b, including all available measurements from the literature. Overplotted are three models for the atmosphere of GJ1214b: a hydrogen dominated atmosphere with solar composition (green line), a hydrogen dominated atmosphere with clouds and low methane abundance (red line) and a water dominated atmosphere (blue line). The dashed curves at the bottom are the transmission curves for the different filters. De Mooij et al. (2012).

These results correlate best with the 'waterworld' model of GJ 1214b, which de Mooij et al. conclusede seems the most likely, given the available data.

See also Kepler 69c; a Super Venus rather than a Super Earth? Questioning the possible planet of Alpha Centauri, Two new more planets discovered in the Gliese 676 system, The peculiar planets of Kepler-36 and The atmosphere of GJ1214b.

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The atmosphere of GJ1214b.

The planet GJ1214b was discovered in December 2009 by the MEarth Project at Fred Lawrence Whipple Observatory in Arizona and described in a paper in the journal Nature. It orbits the star GJ1214A (A implies the first body in a system, B the second; stars are capitalized, planets are not), approximately 40 light years from Earth in the constellation of Ophiuchus. GJ1214A is a cool Red Dwarf star, only one fifth the size of our sun and 0.3% as bright. GJ1214b orbits this star every 38 hours, at 1.4% the distance at which our planet orbits the sun.

An artists impression of GJ1214b and it's star.

The planet was discovered by the tidal effect it exerts upon its star; a large planet orbiting close to a small star exerts a considerable gravitational pull, causing the star to wobble back and forth as the planet orbits it. This can be detected by sensitive telescopes using the Doppler effect, as the star moves towards us the light waves it emits are slightly compressed, making it appear slightly blueish (to very sensitive spectrometers, not human astronomers), as it moves away the light waves are expanded, making it appear slightly reddish.

Although GJ1214b was discovered by the Doppler method, it also transits (passes in front of) its star, enabling scientists to estimate both the the mass of the planet (by the Doppler method) and its radius (by the dimming it causes when it passes in front of its star) and therefore its density (by multiplying the two). This reveals a planet with some rather unusual properties. GJ1214b is 6.55 times as massive as the Earth, and has a radius 2.678 that of the Earth. This implies a planet with a much lower density than the Earth, which is surprising for a planet this size.

It was initially suggested that the planet could be made up largely of ice, which is far less dense than rock. But the surface temperature of GJ1214b is estimated to be between 399 and 555 K, or 126-282 °C. Another proposal is that GJ1214b might be an ocean planet entirely covered by an ocean hundreds of kilometers deep, with an icy core kept solid by the pressure of the water above it. Water can remain a liquid at temperatures above 100 °C if the pressure is high enough, but the although GJ1214b is big, its low density gives it an estimated gravity of 0.91 that of the Earth, so it is highly unlikely that water could remain a liquid at its surface. It has also been suggested that GJ1214b might be a relatively small (still bigger than the Earth) rocky planet with a thick, dense, atmosphere. But no mechanism has been suggested by which a small, hot, planet could retain such an atmosphere so close to a star, so if this is the case then the planet must be very young, unlikely orbiting a Red Dwarf star estimated to be 6 billion years old, or have moved recently into its current position, which is even harder to explain.

On 28 November 2011 a team led by Zachory Berta of the Harvard-Smithsonian Centre for Astrophysics published a paper on the arXiv online database at Cornell University Library, detailing the results of a spectroscopic study of the atmosphere of GJ1214b using the Wild Field Camera 3 on the Hubble Space Telescope. This found that the upper atmosphere of GJ1214b probably contains a thick layer of cloud, made up of some form of largish molecules, rather than a simple gas such as hydrogen; Berta suggest that this may be water (H₂O).

Any gas made up of molecules containing more than one type of atoms tends to have a greenhouse effect; it absorbs light (energy) at a variety of wavelengths, but emits it in the infra-red part of the spectrum. The obvious model for this in our solar system is Venus, which has an atmosphere containing large amounts of Carbon Dioxide (CO₂) and Sulphuric Acid (H₂SO₄), and a dramatic runaway greenhouse effect. This would suggest that GJ1214b is likely to be hotter, and less Earthlike, than has previously been suggested. The term 'super-Earth' has been used to describe planets of sizes intermediate between Earth and Neptune, but it would appear that in the case of GJ1214b it would appear that 'super-Venus' might be more appropriate.

See also A new study of the HAT-P-13 planetary system, The Keper-18 planetary system, Just how big is Iota Draconis b? and Exoplanets on Sciency Thoughts Youtube.