Exoplanet Discoveries, discussion of the latest finds Options

[alphasam]

Yeah, there is a conference with the press going on now but not being broadcast :-( , the end of the embargo has been set for the evening.

[DEChengst]

ESOcast 87: Pale Red Dot Results:

https://www.eso.org/public/unitedkingdom/videos/eso1629a/

Mass is at least 1.3 Earth masses and the orbital period is 11.2 days.

And the actual press briefing that was given earlier this afternoon.

https://t.co/vC5zgpfB8s

[TheAnt]

Here we got the full facts, a 'year' lasting 11 days, that's a tight orbit for the Proxima C planet ESO press release.

Edit, adding resources that popped up after the embargo was lifted:

The Icecat page The habitability of Proxima Centauri b concludes that the planet is not Earth's twin.

A terrestrial planet candidate in a temperate orbit around Proxima Centauri (PDF)

[Gladstoner]

FYI, Centauri Dreams is a good resource for exoplanets as well as general interstellar exploration topics:

http://www.centauri-dreams.org/

[JRehling]

This work contains, also, an interesting hint of the cup-half-empty: The detection of this planet and not any other apparent signal that interferes appreciably with it says something about a lack of other planets within certain parameters. There must be no planet that is closer to Proxima Centauri and more than a fraction of the mass of Proxima b, nor any planet a bit further out and a few times more massive. This is interesting because, a priori, there could be space for another terrestrial planet with prospects for liquid water on the surface. Proxima Centauri may still – and very likely does – have other planets, but this work places bounds on their size and orbits.

Hopefully, we'll soon get some information on the systems of Alpha Centauri A and Alpha Centauri B. Planets with earthlike temperatures would have much longer orbital periods than 11 days (more like 150 to 500 days) and so much longer studies will be needed. On the plus side, planets orbiting one of those could be observed directly a bit more easily, using a chronograph to block a star that is 0.4 to 1.1 AU away from the planet, which is easier due to the larger angular separation than blocking Proxima Centauri to observe Proxima b.

[Mongo]

Exploring plausible formation scenarios for the planet candidate orbiting Proxima Centauri

We present a study of 4 different formation scenarios that may explain the origin of the recently announced planet `Proxima b' orbiting the star Proxima Centauri. The aim is to examine how the formation scenarios differ in their predictions for the multiplicity of the Proxima planetary system, the water/volatile content of Proxima b and its eccentricity, so that these can be tested by future observations. A scenario of in situ formation via giant impacts from a locally enhanced disc of planetary embryos and planetesimals, predicts that Proxima b will be a member of a multiplanet system with a measurably finite value of orbital eccentricity. Assuming that the local solid enhancement needed to form a Proxima b analogue with a minimum mass of 1.3 Earth masses arises because of the inwards drift of solids in the form of small planetesimals/boulders, this scenario also likely results in Proxima b analogues that are moderately endowed with water/volatiles, arising from the dynamical diffusion of icy planetesimals from beyond the snowline during planetary assembly. A scenario in which multiple embryos form, migrate and mutually collide within a gaseous protoplanetary disc also results in Proxima b being a member of a multiple system, but where its members are Ocean planets due to accretion occurring mainly outside of the snowline, possibly within mean motion resonances. A scenario in which a single accreting embryo forms at large distance from the star, and migrates inwards while accreting either planetesimals/pebbles results in Proxima b being an isolated Ocean planet on a circular orbit. A scenario in which Proxima b formed via pebble accretion interior to the snowline produces a dry planet on a circular orbit. Future observations that characterise the physical/orbital properties of Proxima b, and the multiplicity of the system, will provide valuable insight into its formation history.

The Habitability of Proxima Centauri b I: Evolutionary Scenarios

We analyze the evolution of the potentially habitable planet Proxima Centauri b to identify environmental factors that affect its long-term habitability. We consider physical processes acting on size scales ranging between the galactic scale, the scale of the stellar system, and the scale of the planet's core. We find that there is a significant probability that Proxima Centauri has had encounters with its companion stars, Alpha Centauri A and B, that are close enough to destabilize Proxima Centauri's planetary system. If the system has an additional planet, as suggested by the discovery data, then it may perturb planet b's eccentricity and inclination, possibly driving those parameters to non-zero values, even in the presence of strong tidal damping. We also model the internal evolution of the planet, evaluating the roles of different radiogenic abundances and tidal heating and find that a planet with chondritic abundance may not generate a magnetic field, but all other models do maintain a magnetic field. We find that if planet b formed in situ, then it experienced ~160 million years in a runaway greenhouse as the star contracted during its formation. This early phase may have permanently desiccated the planet and/or produced a large abiotic oxygen atmosphere. On the other hand, if Proxima Centauri b formed with a thin hydrogen atmosphere (<1% of the planet's mass), then this envelope could have shielded the water long enough for it to be retained before being blown off itself. Through modeling a wide range of Proxima b's evolutionary processes we identify pathways for planet b to be habitable and conclude that water retention is the biggest obstacle for planet b's habitability. These results are all obtained with a new software package called VPLANET.

The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present

Proxima b is a planet with a minimum mass of 1.3 MEarth orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the Sun's closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time evolution of the star's spectrum, which is essential for modeling the flux received over Proxima b's lifetime. We also show that Proxima b's obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet's eccentricity and level of triaxiality. Next we consider the evolution of Proxima b's water inventory. We use our spectral energy distribution to compute the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find that Proxima b is likely to have lost less than an Earth ocean's worth of hydrogen before it reached the HZ 100-200 Myr after its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We conclude that Proxima b is a viable candidate habitable planet.

The habitability of Proxima Centauri b II. Possible climates and Observability

Radial velocity monitoring has found the signature of a Msini=1.3~M⊕ planet located within the Habitable Zone of Proxima Centauri, (Anglada-Escud\'e et al. 2016). Despite a hotter past and an active host star the planet Proxima~b could have retained enough volatiles to sustain surface habitability (Ribas et al. 2016).

Here we use a 3D Global Climate Model to simulate Proxima b's atmosphere and water cycle for its two likely rotation modes (1:1 and 3:2 resonances) while varying the unconstrained surface water inventory and atmospheric greenhouse effect.

We find that a broad range of atmospheric compositions can allow surface liquid water. On a tidally-locked planet with a surface water inventory larger than 0.6 Earth ocean, liquid water is always present, at least in the substellar region. Liquid water covers the whole planet for CO2 partial pressures ≳1~bar. For smaller water inventories, water can be trapped on the night side, forming either glaciers or lakes, depending on the amount of greenhouse gases. With a non-synchronous rotation, a minimum CO2 pressure is required to avoid falling into a completely frozen snowball state if water is abundant. If the planet is dryer, ∼0.5~bar of CO2 would suffice to prevent the trapping of any arbitrary small water inventory into polar ice caps. More generally, any low-obliquity planet within the classical habitable zone of its star should be in one of the climate regimes discussed here.

We use our GCM to produce reflection/emission spectra and phase curves. We find that atmospheric characterization will be possible by direct imaging with forthcoming large telescopes thanks to an angular separation of 7λ/D at 1~μm (with the E-ELT) and a contrast of ∼10−7. The magnitude of the planet will allow for high-resolution spectroscopy and the search for molecular signatures.

[JRehling]

An important result, discussed in more detail in the Turbet, et al. paper that Mongo has linked, is that Proxima b will be directly observable and possible to analyze spectrally, using the E-ELT and possibly JWST. Generally speaking, the small separation between HZ planets around M dwarfs will make this impossible for systems located >20 light years away, but we hit the jackpot by getting one at 4.2 light years away, which makes it possible. Other candidate exoplanets that fit the bill for this kind of observation include Kapetyn b (13 light years) and Tau Ceti e (12 light years). The best possible candidates would be any such planets that might exist around Alpha Centauri A or B, which would have a much larger separation from their stars than Proxima b.

So, direct observations may begin in 2019 (JWST) or 2024 (E-ELT), with at least two candidate HZ terrestrial planets to observe, and perhaps many more as the search for more planets continues.

[Mongo]

Tutorial models of the climate and habitability of Proxima Centauri b: a thin atmosphere is sufficient to distribute heat given low stellar flux

Proxima Centauri b, an Earth-size planet in the habitable zone of our nearest stellar neighbour, has just been discovered. A theoretical framework of synchronously rotating planets, in which the risk of a runaway greenhouse on the sunlight side and atmospheric collapse on the reverse side are mutually ameliorated via heat transport is discussed. This is developed via simple (tutorial) models of the climate. These show that lower incident stellar flux means that less heat transport, so less atmospheric mass, is required. The incident stellar flux at Proxima Centauri b is indeed low, which may help enhance habitability if it has suffered some atmospheric loss or began with a low volatile inventory.

[Mongo]

Prospects for Characterizing the Atmosphere of Proxima Centauri b

The newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at 5σ confidence. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.

[Holder of the Tw...]

...the ozone 9.8 micron band could be detected with longer integration times (a few months).

The paper itself mentions 60 days of integration. That's an awful lot of dedicated time on the JWST to ask for.

[JRehling]

I think JWST will be able to study planets in the HZ of nearby G and K stars, but not M dwarfs except for the very closest ones, where the possibility is marginal, and – as you say – very resource-intensive. On the other hand, JWST may be able to do some interesting studies of Proxima b that are less expensive, like directly measuring the temperature.

The Giant Magellan Telescope will have an angular separation similar to JWST, and will probably be useful for more or less the same systems. However, E-ELT will have distinctly better resolution and will be the "killer app" for nearby exoplanet studies.

JWST will give us our first followup studies of the HZ exoplanets with the widest angular separation, such as (the unconfirmed) Tau Ceti e. That may be a very small set. E-ELT will be the next major advance. Then we'll probably have to wait for a new space-based telescope designed for the task to improve on that.

[katodomo]

As an overall strategy, JWST is more of an intermediate and supplanting step:

On the European side, PLATO is supposed to launch around the same time that E-ELT becomes active (and around the same time GAIA's catalogue gets published), which will probably make a great pair in space-based detection and first characterization and then ground followup observation throughout the second half of the 2020s.

The US side similarly works with TMT and the still only proposed WFIRST and ATLAST missions in the same timeframe.

[ZLD]

Not getting my hopes up for TMT anymore. While only a little over half the light gathering ability, the Giant Magellan is trucking right along though and is supposed to be done in less than 5 years from now.

[Mongo]

This one is interesting, it considers what can be accomplished with the existing ESO VLT observatory with feasible upgrades to operational (SPHERE) and under-construction (EXPRESSO) instruments:

Atmospheric characterization of Proxima b by coupling the SPHERE high-contrast imager to the ESPRESSO spectrograph

Context. The temperate Earth-mass planet Proxima b is the closest exoplanet to Earth and represents what may be our best ever opportunity to search for life outside the Solar System.

Aims. We aim at directly detecting Proxima b and characterizing its atmosphere by spatially resolving the planet and obtaining high-resolution reflected-light spectra.

Methods. We propose to develop a coupling interface between the SPHERE high-contrast imager and the new ESPRESSO spectrograph, both installed at ESO VLT. The angular separation of 37 mas between Proxima b and its host star requires the use of visible wavelengths to spatially resolve the planet on a 8.2-m telescope. At an estimated planet-to-star contrast of ~10^-7 in reflected light, Proxima b is extremely challenging to detect with SPHERE alone. The use of the high-contrast/high-resolution technique can overcome present limitations by combining a ~10^3-10^4 contrast enhancement from SPHERE to a ~10^4 gain from ESPRESSO.

Results. We find that significant but realistic upgrades to SPHERE and ESPRESSO would enable a 5-sigma detection of the planet and yield a measurement of its true mass and albedo in 20-40 nights of telescope time, assuming an Earth-like atmospheric composition. Moreover, it will be possible to probe the O2 bands at 627, 686 and 760 nm, the water vapour band at 717 nm, and the methane band at 715 nm. In particular, a 3.6-sigma detection of O2 could be made in about 60 nights of telescope time. Those would need to be spread over 3 years considering optimal observability conditions for the planet.

Conclusions. The very existence of Proxima b and the SPHERE-ESPRESSO synergy represent a unique opportunity to detect biosignatures on an exoplanet in the near future. It is also a crucial pathfinder experiment for the development of Extremely Large Telescopes and their instruments (abridged).

From the paper's conclusions:

– We find that the reflected spectrum from Proxima b can be detected at the 5- level in 20-40 nights of telescope time for a contrast enhancement factor K = 3000 (SPHERE+) and a planet-to-star flux ratio of 1.0-1.4 x 10^7 (Earth-like atmospheres). This includes a measurement of the planet true mass (as opposed to minimum mass) and orbital inclination, and the measurement of its broadband albedo.

– We find that O2 can be detected at the 3.6- level in about 60 nights of observing time at K = 5000, for a planet-to-star contrast of 1.4 x 10^7. Those nights would need to be spread over 3 years to guarantee optimal observability conditions of the planet and sucient separation between telluric and planetary O2 lines.

– We also show that H2O can be probed in a similar amount of telescope time provided the H2O column density is similar to wet regions of Earth.

– Finally, it is likely that CH4 is detectable as well if its column density is similar to or larger than the one seen in Jupiter and Saturn, although we could not address this point quantitatively.

In conclusion, while we do not underestimate the technical challenges of our proposed approach, we do believe that SPHERE+ESPRESSO is competitive for becoming the first instrument to characterize a habitable planet.

[Explorer1]

Press conference tomorrow on a new discovery, experts of exoplanet atmospheres involved: https://www.nasa.gov/press-release/nasa-to-...r-solar-system/

Nasawatch.com has some good guesswork of what it may be...