NGTS-1b, a Jovian Planet in a Red Darf star system.

Red Dwarfs (or M-Dwarfs), stars with a surface temperature of less than 4000 K (compared to 5778 K for our Sun), which typically indicates a star with half the mass of our Sun or less, are the most abundant stars in the Universe, and of great interest to astronomers searching for exoplanets. Because Red Dwarfs are small, any planets orbiting them will be proportionately large, making them easier to detect by both the amount of light it blocks as it passes in front of the star, and by the extent which they move the host star as they orbit it (strictly speaking, both a star and a planet gravitationally bound to it orbit their common centre of gravity; for most planets this has very little effect, as that centre of gravity will be very close to the centre of the star, but when planets have masses that are significant proportions of those of the stars they orbit, then the planet can cause the star to wobble to a significant degree, making it possible for astronomers to detect it). The small size of Red Dwarfs also means that their habitable zone (zone in which planets might have liquid water on their surface, making them potentially habitable for recognisable life-forms) is closer to the star, where planets are more easily detected. Most planets discovered in Red Dwarf systems are quite small, with masses four times that of the Earth or less, which is interesting, as most planets discovered around larger stars are Jovian (close in size to the planet Jupiter) or SuperJovian (significantly larger than Jupiter) worlds, making it possible that the formation of such large planets around small stars is somehow more difficult than around larger stars.

In a paper published on the arXiv database at Cornell University Library on 30 October 2017, and  accepted for publication in the Monthly Notices of the Royal Astronomical Society, a team of scientists led by Daniel Bayliss of the Observatoire de Gen eve at the Universit e de Gen eve, describe the discovery of a Jovian planet orbiting the Red Dwarf Star 2MASS J05305145-3637508, 600 light years from Earth in the constellation of Columba.

The discovery was made by the Next Generation Transit Survey (NGTS), a fully automated array of twelve 20 cm aperture Newtonian telescopes situated at the ESO Paranal Observatory in Chile, and the star system is renamed NGTS-1 (i.e. the first planet hosting star system discovered by the Next Generation Transit Survey), with the star becoming NGTS-1A and the planet NGTS-1b (when naming objects in other stellar systems stars are indicated with an upper case letter, and planets with a lower case letter).

An artists impression of the NGTS stellar system. Mark Garlick/University of Warwick.

NGTS-1A has an effective surface temperature of 3916 K, a mass 0.617 times that of our Sun and a radius 0.573 times the Sun's. This star is orbited every 2.65 days by NGTS-1b, a planet at a distance of 0.326 AU (i.e. 0.326 times as far from the star as the Earth is from the Sun) with a mass 0.812 times that of Jupiter, and a radius 1.33 times Jupiter's; this larger radius being due to the high temperature of the planet, which has a surface temperature of 790 K, due to its close proximity to its star.

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Planets in the NY Virginis system.

NY Virginis is an eclipsing binary system roughly 2000 light years from the Earth in the constellation of Virgo. The name NY Virginis implies the 258th variable star in the constellation of Virgo. The system is also known as PG1336-018, where the PG stands for 'Palomar Green'; it was discovered during the Palomar Green survey.

The system is made up of two stars referred to as M₁ and M₂. M₁ has a mass of approximately 46% of that of our sun, but is over six times as hot (33 000 K as opposed to 5578 K for our sun). It is an 'Extreme Horizontal Branch' blue-white subdwarf-star, where the 'Extreme Horizontal Branch' refers to the position on a graph where colour/temperature is plotted against temperature. Stars of this type have run out of hydrogen in their core and expanded to form Red Giant stars, in which helium is fused in the core and hydrogen in the outer layers, then lost these outer layers for some reason (probably in this case interaction with M₂), exposing the helium-fusing core.

Orbiting this at a distance of slightly under 5 million km is M₂, a Red Dwarf star with 14% of the mass of our sun and a temperature of about 3000 K. Red Dwarfs are small, cool stars which do not get particularly hot due to their low mass, but subsequently burn for much longer than larger stars.

This month a study of the system by a team lead by SB Qian of the Yunnan Observatory was published in a paper on the arXiv database at Cornell University Library. Qian et al. combined data from an earlier study of the system by the South African Astronomical Observatory with new data collected by the Jorge Sahade Telescope in Argentina and the Yunnan Observatory.

This study was able to detect irregularities in the orbit of the two stars, which they used to calculate the existence of a planet with a mass of 2.3 times that of Jupiter, orbiting at a distance of 3.3 AU, that is to say 3.3 times the distance at which the Earth orbits the Sun, or twice the distance at which Mars orbits, every 2900 days. Qian et al. refer to this planet as M₃, but it is referred to elsewhere as NY Virginis b, or NY Vir b. This is inaccurate; using conventional numbering for the system M₁ would be NY Virginis A, M₂ would be NY Virginis B and M₃ would be NY Virginis c; since all bodies in the system are lettered, stars with a capitol and planets with a lower case letter.

The inclusion of M₃ in the model still does not completely resolve the irregularities in the two stars orbits. Qian et al. therefore infer a second planet, with a mass of about 2.5 times that of Jupiter, orbiting roughly once every 15 Earth years.

An artist's impression of a binary system with two planets. By scientific illustrator Mark A. Garlick.

Qian et al. also attempt to model the history of the NY Virginis system. As noted above, M₂ orbits M₁ at a distance of only 5 million km, but it cannot have been this close throughout the history of the system. Qian et al. calculate that if M₁ had originally had a mass the same as our sun (which is an arbitrary figure, but works for modeling the evolution of the system), then it would have grown till its radius was roughly 0.5 AU - half the radius of Earth's orbit, or slightly greater than the orbit of Mercury. In this model if M₂ had originally orbited M₁ at a distance of 0.8 AU (a slightly greater distance than that at which Venus orbits the sun) then once M₁ reached a radius of 0.5 AU, then M₂ would have started to tear away the outer atmosphere of M₁. This would have slowed down M₂ in its orbit, causing it to spiral inwards towards M₁, stabilizing in its current orbit once the outer atmosphere of M₁ was used up.

See also Kepler discovers two Cthonian planets orbiting a subdwarf star, Kepler-16 (AB)b and Exoplanets on Sciency Thoughts YouTube.

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.