Using TESS observations from 2 sectors, ground-based follow-up from LCOGT and NGTS, and space-based follow-up from Spitzer, a team of international astronomers reported the discovery of three new planets TOI-431 b, c, d orbiting a K-dwarf star TOI-431. Their study recently appeared in Arxiv.
The TOI-431 system was observed in TESS Sectors 5 (Nov 15 to Dec 11 2018) and 6 (Dec 15 2018 to Jan 6 2019) on Camera 2 in the 2-minute cadence mode (𝑡exp = 2 min). TOI-431.01 (now TOI-431 d) was flagged on Feb 8 2019 by the MIT Quick-Look Pipeline with a signal-to-noise ratio (SNR) of 58 while, TOI-431.02 (now TOI-431 b) was flagged later, on June 6, after identification by the TESS Science Processing Operations Center (SPOC) pipeline with an SNR of 24 in a combined transit search of Sectors 5-6.
Now, using ground-based follow-up from LCOGT and NGTS, and space-based follow-up from Spitzer, a team of international astronomers led by Ares Osborn confirmed the planetary nature of TOI-431 d and b.
They also characterized the bright, multi-planet system TOI-431 using photometry and radial velocities and found evidence that the host star is rotating with a period of 30.5 days.
“The photometric data was modelled jointly with RV data from the HARPS spectrograph, and further RVs from iSHELL, FEROS, and Minerva-Australis are included in our analysis.”
Additionally, it has been shown that the TOI-431 b is a super-Earth, with an ultra-short period of 0.49 days, a radius of 1.28 R, a mass of 3.07 M and a density of 8 g cm³. It likely has a negligible envelope due to substantial atmosphere evolution via photoevaporation, and an Earth-like composition.
While, TOI-431 d is a sub-Neptune with a period of 12.46 days, radius of 3.29 R, a mass of 9.90 M and density of 1.36 g cm. It has likely retained a substantial H-He envelope of about 4 per cent of its total mass.
Moreover, they discovered TOI-431 c in the HARPS RV data, but is not seen to transit in the TESS light curves. It has a period of 4.84 days and a minimum mass similar to the mass of TOI-431 b; extrapolating this minimum mass to a radius via the MR relation places it as a likely second super-Earth.
Finally, it has been suggested that, TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterisation, while the super-Earth TOI-431 b may be a stripped core.
“This system is a candidate for further study of planetary evolution, with TOI-431 b and d either side of the radius valley. The system is bright, making it amenable to follow-up observations. TOI-431 b, in particular, would potentially be an interesting target for phase-curve observations with JWST.”
Reference: Ares Osborn, David J. Armstrong, Bryson Cale, Rafael Brahm, Robert A. Wittenmyer, Fei Dai, Ian J. M. Crossfield, Edward M. Bryant, Vardan Adibekyan, Ryan Cloutier, Karen A. Collins, E. Delgado Mena, Malcolm Fridlund, Coel Hellier, Steve B. Howell, George W. King, Jorge Lillo-Box, Jon Otegi, S. Sousa, Keivan G. Stassun, Elisabeth C. Matthews, Carl Ziegler, George Ricker, Roland Vanderspek, David W. Latham, S. Seager, Joshua N. Winn, Jon M. Jenkins, Jack S. Acton, Brett C. Addison, David R. Anderson, Sarah Ballard, David Barrado, Susana C. C. Barros, Natalie Batalha, Daniel Bayliss, Thomas Barclay, Björn Benneke, John Berberian Jr., Francois Bouchy, Brendan P. Bowler, César Briceño, Christopher J. Burke, Matthew R. Burleigh, Sarah L. Casewell, David Ciardi, Kevin I. Collins, Benjamin F. Cooke, Olivier D. S. Demangeon, Rodrigo F. Díaz, C. Dorn, Diana Dragomir, Courtney Dressing, Xavier Dumusque, Néstor Espinoza, P. Figueira, Benjamin Fulton, E. Furlan, E. Gaidos, C. Geneser, Samuel Gill, Michael R. Goad, Erica J. Gonzales, Varoujan Gorjian, Maximilian N. Günther, Ravit Helled, Beth A. Henderson, Thomas Henning, Aleisha Hogan, Saeed Hojjatpanah, Jonathan Horner, Andrew W. Howard, Sergio Hoyer, Dan Huber, Howard Isaacson, James S. Jenkins, Eric L. N. Jensen, Andrés Jordán, Stephen R. Kane, Richard C. Kidwell Jr., John Kielkopf, Nicholas Law, Monika Lendl, M. Lund, Rachel A. Matson, Andrew W. Mann, James McCormac, Matthew W. Mengel, Farisa Y. Morales, Louise D. Nielsen, Jack Okumura, Hugh P. Osborn, Erik A. Petigura, Peter Plavchan, Don Pollacco, Elisa V. Quintana, Liam Raynard, Paul Robertson, Mark E. Rose, Arpita Roy et al., “TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet”, Arxiv, pp. 1-21, 2021. https://arxiv.org/abs/2108.02310
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The Institute of Astrophysics of Andalusia (IAA-CSIC) leads the detection of what, according to the data, is the most common type of planetary systems around dwarf stars, the most common in the Milky Way
The era of the detection of planets outside our Solar System, which began less than three decades ago, has so far yielded more than four thousand detected planets. Their astonishing variety has shown that the structure of our Solar System, with rocky planets in the inner regions and gaseous icy planets in the outer regions, is not as typical as previously thought, and that other configurations appear more common, such as gas giant planets very close to their stars or systems with several super-Earths around dwarf stars. In this context, a new detection of two planetary systems by the CARMENES instrument, operating at Calar Alto Observatory (CAHA, Almería), reinforces the idea that dwarf stars tend to harbour rocky planets.
“Our current understanding of the formation of low-mass planets in orbits very close to small stars suggests that they are very abundant, with an average of at least one planet per star. Despite this abundance, we hardly have any data on the density of these planets that would allow us to deduce their composition,” says Pedro J. Amado, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who heads the study.
In our Solar System, Earth, Mars, Mercury and Venus are categorised as terrestrial or rocky planets. For extrasolar planets, those between half and twice the size of Earth are considered to be terrestrial, while those up to ten times the mass of Earth are classified as super-Earths, terms that have no implications for surface conditions or habitability. In fact, while the composition of Earth-like exoplanets may be similar to that of the rocky planets in the Solar System, the composition of super-Earths may also include other combinations of gas, rock, ice or water.
The newly detected systems are found around the red dwarf (or M-dwarf) stars G 264-012 and Gl 393. Two planets with a minimum mass of 2.5 and 3.8 times that of the Earth have been found around the former, orbiting their star every 2.3 and 8.1 days. The planet in Gl 393 has a minimum mass of 1.7 Earth masses and orbits its star every seven days. All three planets fall into the category of hot earths and super-earths, and reach temperatures that preclude the presence of liquid water on the surface.
“To understand how the different planetary systems we are observing form and evolve, we need robust statistics on the number of planets that exist, as well as information on the architecture of the systems and the density of the planets. This will allow us to explain those that do not fit the known mechanisms, such as the GJ 3512 system that we also found with CARMENES, which has a giant planet around a dwarf star, or to confirm the tendency of dwarf stars to host multiple systems,” says Pedro J. Amado (IAA-CSIC).
In this sense, this work has also made it possible to detect a new factor that seems to influence the detections, since the planet around the star Gl 393 had gone unnoticed in previous campaigns with highly efficient planet-hunting instruments. Red dwarf stars show intense activity in the form of flares that can mask the signal from potential planets, and the research team noted that the non-detection of Gl 393’s planet could be due to the fact that previous observations were carried out during a peak of activity. They concluded that planets can most easily be detected in dwarf stars with moderate activity or outside the peaks of the star’s activity cycle.
“The CARMENES work is focused on extending the available data to compose a global picture of the planetary systems. The three planets in these two systems are among the smallest in mass, and therefore in the amplitude of the radial velocity they instill in their stars, which accounts for the quality of the instrument,” concludes Pedro J. Amado (IAA-CSIC).
Reference: P. J. Amado et al. “The CARMENES search for exoplanets around M dwarfs. Two terrestrial planets orbiting G264-012 and one terrestrial planet orbiting Gl393”. Astronomy & Astrophysics, https://doi.org/10.1051/0004-6361/202140633, June 2021
Ultra-short-period (USP) planets refer to a class of exoplanets (usually radius smaller than 2 R) with periods less than 1.0 day. Since the earliest examples were discovered back in the late 2000’s, more than 100 such USP planets have been reported to date. Now, a team of international astronomers presented observations of two bright M dwarfs (TOI-1634 and TOI-1685: J = 9.5 − 9.6) hosting ultra-short period (USP) planet candidates, identified by the TESS mission. The two stars are similar in temperature, mass, and radius, and the planet candidates are both super-Earth-sized (1.25 R < Rp < 2.0 R). For both systems, light curves from the ground-based photometry exhibit planetary transits, whose depths are consistent with those by the TESS photometry.
They also refined the transit ephemerides based on the ground-based photometry, finding the orbital periods of P = 0.9893457 day and P = 0.6691425 day for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using the IRD spectrograph on the Subaru 8.2m telescope, they confirmed the planetary nature of the TOIs, and measure their masses: 9.94 M and 3.84 M for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model.
Combining those with the planet radii of Rp = 1.773 R (TOI-1634b) and 1.463 R (TOI-1685b), they found that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets.
They concluded that, TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of testing the formation scenarios for USP planets. While, excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.
Reference: Teruyuki Hirano, John H. Livingston, Akihiko Fukui, Norio Narita, Hiroki Harakawa, Hiroyuki Tako Ishikawa, Kohei Miyakawa, Tadahiro Kimura, Akifumi Nakayama, Naho Fujita, Yasunori Hori, Keivan G. Stassun, Allyson Bieryla, Charles Cadieux, David R. Ciardi, Karen A. Collins, Masahiro Ikoma, Andrew Vanderburg, Thomas Barclay, C. E. Brasseur, Jerome P. de Leon, John P. Doty, René Doyon, Emma Esparza-Borges, Gilbert A. Esquerdo, Elise Furlan, Eric Gaidos, Erica J. Gonzales, Klaus Hodapp, Nobuhiko Kusakabe, Masayuki Kuzuhara, David Lafrenière, David W. Latham, Bob Massey, Mayuko Mori, Felipe Murgas, Jun Nishikawa, Taku Nishiumi, Masashi Omiya, Martin Paegert, Enric Palle, Hannu Parviainen, Samuel N. Quinn, Steve B. Howell, Keisuke Isogai, Shane Jacobson, Jon M. Jenkins, Eric L. N. Jensen, Kiyoe Kawauchi, Takayuki Kotani, Tomoyuki Kudo, Seiya Kurita, Takashi Kurokawa, George R. Ricker, Richard P. Schwarz, Sara Seager, Peter Tenenbaum, Yuka Terada, Roland K. Vanderspek, Noriharu Watanabe, Joshua N. Winn, “Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions”, ArXiv, pp. 1-27, 2021. https://arxiv.org/abs/2103.12760
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“Super-Earth” planets are giant-size versions of Earth, and many researches suggested that they’re more likely to be habitable than Earth-size worlds. But, a study done by Hippke and colleagues revealed how difficult it would be for any aliens on these exoplanets to explore space.
Do we inhabit the best of all possible worlds? From a variety of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable. Other, more habitable (“superhabitable”) worlds might exist. Planets more massive than Earth can have a higher surface gravity, which can hold a thicker atmosphere, and thus better shielding for life on the surface against harmful cosmic rays. Increased surface erosion and flatter topography could result in an “archipelago planet” of shallow oceans ideally suited for biodiversity. There is apparently no limit for habitability as a function of surface gravity as such. Size limits arise from the transition between Terran & Neptunian worlds around 2 ± 0.6 R. The largest rocky planets known so far are ∼ 1.87 R⊕, ∼ 9.7 M⊕ (Kepler-20 b, Buchhave et al. 2016). When such planets are in the habitable zone, they may be inhabited by “Super-Earthlings” (SEALs). Can Seals still use chemical rockets to leave their planet?
At our current technological level, spaceflight requires a rocket launch to provide the thrust needed to overcome Earth’s force of gravity. Chemical rockets are powered by exothermic reactions of the propellant, such as hydrogen and oxygen. Other propulsion technologies with high specific impulses exist, such as nuclear thermal rockets (e.g., NERVA, Arnold & Rice 1969), but have been abandoned due to political issues. Rockets suffer from Tsiolkovsky’s equation (Tsiolkovsky 1903): if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive. While Hippke and colleagues hand-wave away many things in this paper, they do respect rocket science.
State-of-the-art technology such as the recently introduced Falcon Heavy has a rocket height of 70 m, mass of 1421 t, and delivers a payload of 16.8 t to an Earth escape velocity, so that the payload fraction is ∼ 1 %. Hippke and colleagues now explored how much rocket is needed for planets with higher surface gravity.
To explore this they employed a method which I explained below:
The achievable maximum velocity change of a chemical rocket is
where, m0 is the initial total mass (including fuel), mf is the final total mass without fuel (the dry mass), and vex is the exhaust velocity. Here, we can substitute vex = g0 Isp where g0 = G M⊕/R² ∼ 9.81 ms-¹ is the standard gravity and Isp is the specific impulse (total impulse per unit of propellant), typically ∼ 350 . . 450 s for hydrogen/oxygen.
To leave Earth’s gravitational influence, a rocket needs to achieve at minimum the escape velocity
for Earth, and vesc ∼ 27.1 kms-¹ for a 10 M, 1.7 R Super-Earth similar to Kepler-20 b.
They consider a single-stage rocket with Isp = 350 s and wish to achieve ∆v > vesc. The mass ratio of the vehicle becomes
which evaluates to a mass ratio of ∼ 26 on Earth, and ∼ 2,700 on Kepler-20 b. Consequently, a single-stage rocket on Kepler-20 b must burn 104× as much fuel for the same payload (∼ 2,700 t of fuel for each t of payload). This example neglects the weight of the rocket structure itself, and is therefore a never achievable lower limit. In reality, rockets are multistage, and have typical mass ratios (to Earth escape velocity) of 50 . . . 150. For example, the Saturn V had a total weight of 3,050 t for a lunar payload of 45 t, so that the ratio is 68. The Falcon Heavy has a total weight of 1,400 t and a payload of 16.8 t, so that the ratio is 83.
For a mass ratio of 83, the minimum rocket (1 t to vesc) would carry 9,000 t of fuel on Kepler-20 b, which is only 3× larger than a Saturn V (which lifted 45 t). To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships. They showed such a rocket to scale in Figure 1. For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ∼ 400,000 t. Researchers are not sure how ridiculous such a rocket is, because it is still less heavy than the Pyramid of Cheops, although not by much.
So, as now you know how big Rockets aliens need. Lets see, how would different worlds launch their chemical rockets.
Friends, rockets work better in space than in an atmosphere. How about launching the rocket from a high mountain? At first glance, this is a great idea, because the rocket thrust is given by
where ˙m is the mass flow rate, Ae is the cross-sectional area of the exhaust jet, P1 is the static pressure inside the engine, and P2 is the atmospheric pressure. The exhaust velocity is maximized for zero atmospheric pressure, i.e. in vacuum. Unfortunately, the effect is not very large in practice. For the Space Shuttle’s main engine, the difference between sea level and vacuum is ∼ 25 %. Atmospheric pressure below 0.4 bar (Earth altitude 6,000 m) is not survivable long term for humans, and presumably neither for Seals. Then, the effect is ∼ 15%. Such low pressures are reached in lower heights on Super-Earths, because the gravity pulls the air down. Strongly.
Another effect which is to the SEALs disadvantage is that the bigger something is, the less it can deviate from being smooth. Tall mountains will crush under their own weight (the “potato radius” is ∼ 238 km). Therefore, Hippke and colleagues expect more massive planets to have smaller mountains. This will be detectable through transit observations in future telescopes.
Indeed, the largest mountains in our solar system are on less massive bodies. Researchers recommend that the SEALs use shovels to make a gigantic mountain, exceeding the atmosphere, and launch their rocket from the vacuum on top. Researchers encourage further research in this rather under-explored field.
Launching rockets from water-worlds
Many habitable (and presumably inhabited) planets might be waterworlds, & intelligent life in water & sub-surface is plausible. How would Nautical Super Earthlings (Navy SEALs) launch their rockets? This is actually less absurd than most other things in this paper, but harder than the reader might think.
An elegant method would be to build an Alien “megastructure”, as postulated by Wright in 2016. To launch the rocket, a large floating structure can be used. Turtles do not exist in such sizes, but nautical floats can be built in turtle shapes. The rocket would be placed on the turtle’s shield (out of the water), dried with towels, and then launched towards the heavens, while other (living) turtles spray water towards the launch pad turtle to cool down the hot exhaust fumes.
These minor aquatic launch complications make the theory of oceanic rocket launches appear at first quite alien; presumably land-based launches seem equally human to alien rocket scientists.
Launching rockets on worlds with an icy crust
Subsurface liquid water oceans exist below the frozen surfaces of Enceladus and Europa, and it appears plausible that such worlds are habitable. How would ice nautical Super-Earthlings (iNavy SEALS) launch their chemical rockets? They need an icebreaker.
One method which works (sometimes not so well) is to use classical explosives to flash-vaporize water into steam. The pressure of the expanding gas drives the missile upwards in a tube. This works well for comparably small ICBMs launched from submerged submarines, but these have no issues with ice coverage. As a minor annoyance, ice crusts on Europa and Enceladus are tens of kilometers thick. A series of fusion bombs could be used to blast through the ice and then, quickly, lift the massive rocket completely out of the water. This is highly non-trivial, because of the vacuum on the outside, which does not allow for a liquid phase of the water. The expanding vaporized fountain would re-freeze quickly, leaving little time for the journey.
Fortunately for the fellow iNavy-SEALS who must stay behind, water (H2O) cannot become radioactive itself, and radioactive particles are mostly not soluble in water. Therefore, they can be filtered out after the launch. Typical fallout particles sink to the sea floor in a few days, and in the meantime, drinking water can be drawn from near the top of the pool. Among the authors who have not pointed this out is Einstein (1905).
If there is no plutonium on the iNavy-SEALS’ world to build atomic bombs, researchers recommend to use “Occam’s Laser” to blast a hole in the ice. Earthlings will use it for ”Breakthrough Starshot”, a mission to α Cen. A km-sized phased aperture would emit 100 GW of laser power, sufficient to accelerate a 1 g “space-chip” to v = 0.2 c in minutes. Light sails are no rockets, and therefore not rocket science, therefore researchers recommend to use the laser to blast through the ice.
Another way to get out is to ascend through the plumes which spew from Enceladus’ south polar surface. For details, please refer the fig given below and Jules Verne (1864).
So, what is the amount of fuel required for different surface gravities?
Well, according to researchers, for a payload of one ton to escape velocity, the required amount of chemical fuel is ∼ 3.3 exp(g0). The situation is not that bad for medium-sized Super-Earths, but quickly escalates due to the nasty exponential function (who likes these anyways?). On worlds with a surface gravity of ≥ 10 g0, a sizable fraction of the planet needs to be used up as chemical fuel per launch, limiting the total number of flights. They showed in Figure 3 how ridiculous the amount of fuel is for worlds with even higher surface gravity. On such worlds it is cheaper to destroy the planet rather than convert it into fuel.
In the ultimate limit, we may use the whole mass of the universe (ordinary matter only) of ≈ 10^50 kg as oxygen/hydrogen fuel. Such a chemical rocket can overcome a surface gravity of ∼ 35.3 g0. For comparison, a neutron star’s density results in a very high surface gravity of ≈ 10^11g0. Pulsar-lings will thus not become chemically space-faring beings. If such a “universal chemical rocket” is launched from space directly, its final velocity would be ∼ 400 km s-¹, or ∼ 0.13% the speed of light. It has no trouble with interstellar dust, because its road to nowhere is free.