Author Topic: Exoplanets And Stars Thread  (Read 17977 times)

Offline Bynaus

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Re: Exoplanets And Stars Thread
« Reply #140 on: 08/09/2017 08:34 PM »
It is noteworthy that if the planetary system is aligned with the debris disk (which seems very likely), the true masses of the four planets are about double their minimum masses. So the inner two planets would be large super-Earths, whereas the outer two would be mini-Neptunes.

@StarOne: I think of these three stars, Alpha Cen A is most sun-like. It is also a G2V type, is only 10% more massive and has a similar rotation period. Then Tau Ceti, then Alpha Cen B.

@Bob Shaw: the planets that van de Kamp thought were there (in the 1960ies) do not exist, this we know for sure. Otherwise, we only have upper limits on mass/distance (<7.5 Earth masses in the habitable zone, e.g.). The Red Dots project is bound to find out!

Thank you. Could you direct me towards any articles or papers speculating on the origins of the debris disk around Tau Ceti?

Look here for a recent overview of the situation (although still with the originally proposed 5 planets):
http://adsabs.harvard.edu/abs/2015csss...18..783L

The classic paper on the Tau Ceti debris disk is probably this one: http://adsabs.harvard.edu/abs/2004MNRAS.351L..54G

Offline Star One

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Exoplanets And Stars Thread
« Reply #141 on: 08/09/2017 08:38 PM »
It is noteworthy that if the planetary system is aligned with the debris disk (which seems very likely), the true masses of the four planets are about double their minimum masses. So the inner two planets would be large super-Earths, whereas the outer two would be mini-Neptunes.

@StarOne: I think of these three stars, Alpha Cen A is most sun-like. It is also a G2V type, is only 10% more massive and has a similar rotation period. Then Tau Ceti, then Alpha Cen B.

@Bob Shaw: the planets that van de Kamp thought were there (in the 1960ies) do not exist, this we know for sure. Otherwise, we only have upper limits on mass/distance (<7.5 Earth masses in the habitable zone, e.g.). The Red Dots project is bound to find out!

Thank you. Could you direct me towards any articles or papers speculating on the origins of the debris disk around Tau Ceti?

Look here for a recent overview of the situation (although still with the originally proposed 5 planets):
http://adsabs.harvard.edu/abs/2015csss...18..783L

The classic paper on the Tau Ceti debris disk is probably this one: http://adsabs.harvard.edu/abs/2004MNRAS.351L..54G

Thanks that second paper sounds interesting.

Seems likely any life that might have got going would probably have been 'bombed' out of existence by that debris disk.
« Last Edit: 08/09/2017 08:40 PM by Star One »

Online Dao Angkan

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Re: Exoplanets And Stars Thread
« Reply #142 on: 08/09/2017 08:55 PM »
It's only necessary for unicellular photosynthetic organisms to survive long enough that there's enough oxygen in the atmosphere to detect. I'm not convinced that oceans full of unicellular life would be completely wiped out by asteroid and comet impacts.
« Last Edit: 08/09/2017 08:56 PM by Dao Angkan »

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #143 on: 08/09/2017 09:03 PM »
It's only necessary for unicellular photosynthetic organisms to survive long enough that there's enough oxygen in the atmosphere to detect. I'm not convinced that oceans full of unicellular life would be completely wiped out by asteroid and comet impacts.

But with that kind of disc isn't there a danger that the bombardment would be so heavy that it would leave the surfaces of these planets something like IO.

Online Dao Angkan

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Re: Exoplanets And Stars Thread
« Reply #144 on: 08/09/2017 09:18 PM »
It's "only" estimated to be 10x the frequency as on Earth, which might be an issue for complex life, but shouldn't be an issue for unicellular life. I would also guess that super Earths are more likely to have denser atmospheres than Earth, which would help to mitigate the higher frequency of impacts to an extent.

Cometary impacts might also be beneficial for life, bringing water and organics.


Offline Bynaus

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Re: Exoplanets And Stars Thread
« Reply #145 on: 08/09/2017 09:42 PM »
Note that its just that the mass of the debris disk is estimated to be about 10x the mass of the Kuiper belt. To go from there to a 10x higher bombardment rate on the planets in the system is quite problematic. After all, the Earth is primarily bombarded from the asteroid belt, not the Kuiper belt! Bombardment rates on planets primarily depend on the efficiency of delivery of small bodies to planet-crossing orbits (e.g. through resonances and non-gravitational forces), which in turn depends on the system planetary architecture. We know nothing of planets in the outer part (>1 AU) of the Tau Ceti system. Without a detailed simulation, talk of relative bombardment rates and its implications for life on these planets is pure fantasy.

Online Dao Angkan

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Re: Exoplanets And Stars Thread
« Reply #146 on: 08/09/2017 10:34 PM »
Good point, but I still think it's reasonable to assume a higher frequency of impacts. Still, even if that does have an impact on complex life, it's still possible that unicellular photosynthetic life could recover quickly and it might be possible to detect evidence of (or indeed lack of) oxygen in exoplanet atmospheres in the future.




Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #147 on: 08/10/2017 07:27 PM »
Hint of relativity effects in stars orbiting centre of galaxy

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A new analysis of data from ESO’s Very Large Telescope and other telescopes suggests that the orbits of stars around the supermassive black hole at the centre of the Milky Way may show the subtle effects predicted by Einstein’s general theory of relativity. There are hints that the orbit of the star S2 is deviating slightly from the path calculated using classical physics. This tantalising result is a prelude to much more precise measurements and tests of relativity that will be made using the GRAVITY instrument as star S2 passes very close to the black hole in 2018.

https://astronomynow.com/2017/08/10/hint-of-relativity-effects-in-stars-orbiting-centre-of-galaxy/

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #148 on: 08/11/2017 09:24 PM »
Two bright, variable sources with unusual light curves discovered by ASAS-SN

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As part of an ongoing effort by ASAS-SN project (Shappee et al. 2014; Kochanek et al. 2017) to characterize and catalog all bright variable stars, we have discovered two bright, variable sources with unusual light curves---ASASSN-V J033455.88-053957.9 (mean V~13.0) and ASASSN-V J211014.40-242105.3 (mean V~14.5).

ASASSN-V J033455.88-053957.9 raises slowly in brightness by ~0.5 mag in ~1,800 days (ASAS-SN Sky Patrol light curve; data points in red are likely affected by artifacts and should not be taken into consideration). A spectrum of this source was obtained by the RAVE survey (Kunder et al. 2017) giving values of T_eff=3660 K and log g =5.1 cm/s^2, fairly typical for an early-type M-dwarf. The 2MASS colors for this source (J-H=0.59 and H-Ks=0.10) also correspond to M1/M2 spectral type M-dwarf (West et al. 2011). ASASSN-V J033455.88-053957.9 appears in a number of X-ray catalogs, including the Swift X-ray point source catalogue (Evans et al. 2014). ASAS-SN light curve for ASASSN-V J033455.88-053957.9 is very unusual for an M-dwarf, and we encourage further observations of this source.

ASASSN-V J211014.40-242105.3 (ASAS-SN Sky Patrol light curve) is mostly flat at around V~14.5 mag, with a big drop in brightness of at least ~2 mag, lasting at least 150 days. This source was spectroscopically classified as a cataclysmic variable, with Balmer and HeII 4686A emission lines (Kilkenny et al. 2015). We conclude that ASASSN-V J211014.40-242105.3 is most likely a cataclysmic variable (CV) of the VY Scl type.

We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Chinese Academy of Sciences South America Center for Astronomy (CASSACA).

http://www.astronomerstelegram.org/?read=10634

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #149 on: 08/12/2017 07:45 PM »
TRAPPIST-1 is Older Than Our Solar System

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If we want to know more about whether life could survive on a planet outside our solar system, it's important to know the age of its star. Young stars have frequent releases of high-energy radiation called flares that can zap their planets' surfaces. If the planets are newly formed, their orbits may also be unstable. On the other hand, planets orbiting older stars have survived the spate of youthful flares, but have also been exposed to the ravages of stellar radiation for a longer period of time.

Scientists now have a good estimate for the age of one of the most intriguing planetary systems discovered to date -- TRAPPIST-1, a system of seven Earth-size worlds orbiting an ultra-cool dwarf star about 40 light-years away. Researchers say in a new study that the TRAPPIST-1 star is quite old: between 5.4 and 9.8 billion years. This is up to twice as old as our own solar system, which formed some 4.5 billion years ago.

The seven wonders of TRAPPIST-1 were revealed earlier this year in a NASA news conference, using a combination of results from the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, NASA's Spitzer Space Telescope, and other ground-based telescopes. Three of the TRAPPIST-1 planets reside in the star's "habitable zone," the orbital distance where a rocky planet with an atmosphere could have liquid water on its surface. All seven planets are likely tidally locked to their star, each with a perpetual dayside and nightside.

At the time of its discovery, scientists believed the TRAPPIST-1 system had to be at least 500 million years old, since it takes stars of TRAPPIST-1's low mass (roughly 8 percent that of the Sun) roughly that long to contract to its minimum size, just a bit larger than the planet Jupiter. However, even this lower age limit was uncertain; in theory, the star could be almost as old as the universe itself. Are the orbits of this compact system of planets stable? Might life have enough time to evolve on any of these worlds?

"Our results really help constrain the evolution of the TRAPPIST-1 system, because the system has to have persisted for billions of years. This means the planets had to evolve together, otherwise the system would have fallen apart long ago," said Adam Burgasser, an astronomer at the University of California, San Diego, and the paper's first author. Burgasser teamed up with Eric Mamajek, deputy program scientist for NASA's Exoplanet Exploration Program based at NASA's Jet Propulsion Laboratory, Pasadena, California, to calculate TRAPPIST-1's age. Their results will be published in The Astrophysical Journal.

It is unclear what this older age means for the planets' habitability. On the one hand, older stars flare less than younger stars, and Burgasser and Mamajek confirmed that TRAPPIST-1 is relatively quiet compared to other ultra-cool dwarf stars. On the other hand, since the planets are so close to the star, they have soaked up billions of years of high-energy radiation, which could have boiled off atmospheres and large amounts of water. In fact, the equivalent of an Earth ocean may have evaporated from each TRAPPIST-1 planet except for the two most distant from the host star: planets g and h. In our own solar system, Mars is an example of a planet that likely had liquid water on its surface in the past, but lost most of its water and atmosphere to the Sun's high-energy radiation over billions of years.

However, old age does not necessarily mean that a planet's atmosphere has been eroded. Given that the TRAPPIST-1 planets have lower densities than Earth, it is possible that large reservoirs of volatile molecules such as water could produce thick atmospheres that would shield the planetary surfaces from harmful radiation. A thick atmosphere could also help redistribute heat to the dark sides of these tidally locked planets, increasing habitable real estate. But this could also backfire in a "runaway greenhouse" process, in which the atmosphere becomes so thick the planet surface overheats - as on Venus.

"If there is life on these planets, I would speculate that it has to be hardy life, because it has to be able to survive some potentially dire scenarios for billions of years," Burgasser said.

Fortunately, low-mass stars like TRAPPIST-1 have temperatures and brightnesses that remain relatively constant over trillions of years, punctuated by occasional magnetic flaring events. The lifetimes of tiny stars like TRAPPIST-1 are predicted to be much, much longer than the 13.7 billion-year age of the universe (the Sun, by comparison, has an expected lifetime of about 10 billion years).

"Stars much more massive than the Sun consume their fuel quickly, brightening over millions of years and exploding as supernovae," Mamajek said. "But TRAPPIST-1 is like a slow-burning candle that will shine for about 900 times longer than the current age of the universe."

Some of the clues Burgasser and Mamajek used to measure the age of TRAPPIST-1 included how fast the star is moving in its orbit around the Milky Way (speedier stars tend to be older), its atmosphere's chemical composition, and how many flares TRAPPIST-1 had during observational periods. These variables all pointed to a star that is substantially older than our Sun.

Future observations with NASA's Hubble Space Telescope and upcoming James Webb Space Telescope may reveal whether these planets have atmospheres, and whether such atmospheres are like Earth's.

"These new results provide useful context for future observations of the TRAPPIST-1 planets, which could give us great insight into how planetary atmospheres form and evolve, and persist or not," said Tiffany Kataria, exoplanet scientist at JPL, who was not involved in the study.

Future observations with Spitzer could help scientists sharpen their estimates of the TRAPPIST-1 planets' densities, which would inform their understanding of their compositions.

For more information about TRAPPIST-1, visit:

https://exoplanets.nasa.gov/trappist1

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #150 on: 08/12/2017 07:53 PM »
Here's the relevant paper.

On the Age of the TRAPPIST-1 System

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The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928-0502285) hosts a compact system of at least seven exoplanets with sizes similar to Earth. Given its importance for testing planet formation and evolution theories, and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy, we present a comprehensive assessment of the age of this system. We collate empirical age constraints based on the color-absolute magnitude diagram, average density, lithium absorption, surface gravity features, metallicity, kinematics, rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6±2.2 Gyr. The star's color-magnitude position is consistent with it being slightly metal-rich ([Fe/H] ≃ +0.06), in line with its previously reported near-infrared spectroscopic metallicity; and it has a radius (R = 0.121±0.003 R⊙) that is larger by 8-14% compared to solar-metallicity evolutionary models. We discuss some implications of the old age of this system with regard to the stability and habitability of its planets.

https://arxiv.org/abs/1706.02018

Offline Bynaus

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Re: Exoplanets And Stars Thread
« Reply #151 on: 08/14/2017 06:34 AM »
A planetary system around near-by YZ Ceti.

https://arxiv.org/abs/1708.03336

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The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Cet

Exoplanet surveys have shown that systems with multiple low-mass planets on compact orbits are common. Except for few cases, however, the masses of these planets are generally unknown. At the very end of the main sequence, host stars have the lowest mass and hence offer the largest reflect motion for a given planet. In that context, we monitored the low-mass (0.13Msun) M dwarf YZ Cet (GJ 54.1, HIP 5643) intensively and obtained both radial velocities and stellar-activity indicators derived from both spectroscopy and photometry. We find strong evidence that it is orbited by at least three planets in compact orbits (P=1.97, 3.06, 4.66 days), with the inner two near a 2:3 mean-motion resonance. The minimum masses are comparable to that of Earth (Msini=0.75+-0.13, 0.98+-0.14, and 1.14+-0.17 Mearth) and also the lowest masses measured by radial velocity so far. We note the possibility for an even lower-mass, fourth planet with Msini=0.472+-0.096 Mearth at P=1.04 days. An n-body dynamical model is used to put further constraints on the system parameters. At 3.6 parsecs, YZ Cet is the nearest multi-planet system detected to date.

Cool factoid: YZ Ceti is a star currently located only 1.6 LY from Tau Ceti.

Offline Star One

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Exoplanets And Stars Thread
« Reply #152 on: 08/15/2017 07:32 PM »
Tidally locked exoplanets may be more common than previously thought

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Many exoplanets to be found by coming high-powered telescopes will probably be tidally locked — with one side permanently facing their host star — according to new research by astronomer Rory Barnes of the University of Washington.

Barnes, a UW assistant professor of astronomy and astrobiology, arrived at the finding by questioning the long-held assumption that only those stars that are much smaller and dimmer than the sun could host orbiting planets that were in synchronous orbit, or tidally locked, as the moon is with the Earth. His paper, “Tidal Locking of Habitable Exoplanets,” has been accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy.

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Barnes writes: “These results suggest that the process of tidal locking is a major factor in the evolution of most of the potentially habitable exoplanets to be discovered in the near future.”

Being tidally locked was once thought to lead to such extremes of climate as to eliminate any possibility of life, but astronomers have since reasoned that the presence of an atmosphere with winds blowing across a planet’s surface could mitigate these effects and allow for moderate climates and life.

Barnes said he also considered the planets that will likely be discovered by NASA’s next planet-hunting satellite, the Transiting Exoplanet Survey Satellite or TESS, and found that every potentially habitable planet it will detect will likely be tidally locked.

http://www.washington.edu/news/2017/08/14/tidally-locked-exoplanets-may-be-more-common-than-previously-thought/
« Last Edit: 08/15/2017 07:35 PM by Star One »

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #153 on: 08/16/2017 08:00 PM »
Tuning Up RV: A Test Case at Tau Ceti

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And so on. The lesson is clear enough: We have to be extremely careful when interpreting signals below 1 meter per second, the range in which we’ll need to identify Earth-class planets.

But the value of radial velocity is unquestioned. Unlike the transit technique, we don’t have to rely on a fortuitous line-up between a distant planetary system and the Earth — we can therefore extend it to all bright stars of interest. Feng and colleagues think we will be able to use new high precision spectrometers along with these emerging statistical and noise models to find a true Earth analog in the coming decade. Thus this work on Tau Ceti, modeling wavelength-dependent noise, becomes a test case of a new noise model framework that can help us filter background noise out of RV observations.

https://www.centauri-dreams.org/?p=38293

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #154 on: 08/17/2017 03:07 PM »
We may have caught supernova debris slamming into neighboring stars

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In a new paper, a large team of researchers notes that a couple of recent supernovae have shown an unexpected excess of blue light. They then go on to describe the supernova SN 2017cbv, which became visible in March of this year. In this case, the supernova was spotted only a day after the explosion took place. Follow-up observations over the next six weeks allowed the team to build up a detailed record of the aftermath of the supernova (called a light curve). This record was then compared to what we'd expect from a typical type-Ia supernova.

Compared to a standard type-Ia supernova, SN 2017cbv showed an excess of light at all wavelengths, including the blue and UV regions. This was mostly pronounced during the first week after the explosion, as the light ramped up toward its peak. At the peak, there was an excess in redder wavelengths than you'd see in a typical type-Ia supernova.

What could cause this? To find out, the team modified a model of type-Ia supernovae to include a shockwave running into a nearby star. The modified model fits the data well and suggests that somewhere between five and 15 percent of the light during the peak came from the shockwave hitting a nearby star.

The model, however, suggests that the companion star was nearly 40 million miles from the white dwarf (which is a bit closer than Mercury is to our Sun). That's right on the far edge of where you'd expect the companion star to be if the two were close enough to share material. However, the authors point out that this model assumes that they're looking at the event-companion with an ideal viewing angle. If the angle is off, it would look dimmer, which would make the companion star appear to be farther away.

https://arstechnica.com/science/2017/08/we-may-have-caught-supernova-debris-slamming-into-neighboring-stars/

Offline CuddlyRocket

Re: Exoplanets And Stars Thread
« Reply #155 on: 08/18/2017 02:15 AM »
One of the commenters identified the paper on arXiv: Early Blue Excess from the Type Ia Supernova 2017cbv and Implications for Its Progenitor.

Unfortunately, they don't say what happened to the companion star. :(

Offline NIVbV-O77OdV-VSVN-Op-SLE7

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Re: Exoplanets And Stars Thread
« Reply #156 on: 08/18/2017 03:33 AM »
How is it determined if an exoplanet is really orbiting a star?  In other words, there could be some some lost asteroid sized object millions of light years in between us and the star.  Maybe there is an Oort Cloud object eclipsing a star?  It would just periodically move in front of a star to dim the light

Just imagine a boat bobbing on water while watching something on the horizon reappear/disappear.


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Re: Exoplanets And Stars Thread
« Reply #157 on: 08/18/2017 08:13 AM »
How is it determined if an exoplanet is really orbiting a star?  In other words, there could be some some lost asteroid sized object millions of light years in between us and the star.  Maybe there is an Oort Cloud object eclipsing a star?  It would just periodically move in front of a star to dim the light


As far as I can remember, in order for a Kepler candidate to be confirmed as an exoplanet, they must observe 3 periodic eclipses and also be confirmed by a ground based telescope. That might have changed since 2013 though.

Offline gospacex

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Re: Exoplanets And Stars Thread
« Reply #158 on: 08/18/2017 08:36 AM »
How is it determined if an exoplanet is really orbiting a star?  In other words, there could be some some lost asteroid sized object millions of light years in between us and the star.

Asteroid-sized object would not be detectable - the dip is too small.

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  Maybe there is an Oort Cloud object eclipsing a star?  It would just periodically move in front of a star to dim the light

In this case, "periodically" would mean "every 500k years".

Offline Star One

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Re: Exoplanets And Stars Thread
« Reply #159 on: 08/18/2017 06:35 PM »
Dynamics of a Probable Earth-mass Planet in the GJ 832 System

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The stability of planetary orbits around the GJ 832 star system, which contains inner (GJ 832c) and outer (GJ 832b) planets, is investigated numerically and a detailed phase-space analysis is performed. Special attention is given to the existence of stable orbits for a planet less than 15 M ⊕ that is injected between the inner and outer planets. Thus, numerical simulations are performed for three and four bodies in elliptical orbits (or circular for special cases) by using a large number of initial conditions that cover the selected phase-spaces of the planet's orbital parameters. The results presented in the phase-space maps for GJ 832c indicate the least deviation of eccentricity from its nominal value, which is then used to determine its inclination regime relative to the star–outer planet plane. Also, the injected planet is found to display stable orbital configurations for at least one billion years. Then, the radial velocity curves based on the signature from the Keplerian motion are generated for the injected planets with masses 1 M ⊕ to 15 M ⊕ in order to estimate their semimajor axes and mass limits. The synthetic RV signal suggests that an additional planet of mass ≤15 M ⊕ with a dynamically stable configuration may be residing between 0.25 and 2.0 au from the star. We have provided an estimated number of RV observations for the additional planet that is required for further observational verification.

http://iopscience.iop.org/article/10.3847/1538-4357/aa80e2/meta

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