habitable zone

All Your Planets Are Belong to Us

catdynamics — Tue, 12 Mar 2013 08:31:59 +0000

All Your Planets Are Belong to Us

Planets, planets everywhere
and on some there will be drops to drink.

An interesting confluence of research occurred over the last few months, leading to:

"A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around Kepler M-dwarfs" by Ravi kumar Kopparapu PSU (arXiv), ApJLetters in press

Bottom line: about half of low mass stars in the Solar neighbourhood are estimated to have earth size planets in their habitable zone!

The implications are that habitable planets may be very common and the closest one within a few light years.
We'll know for sure very soon.

Schematic of the low star mass habitable zone and current planetary candidates

From PSU Press Release

Last month, two papers were released:

"The Occurrence Rate of Small Planets around Small Stars" by Courtney Dressing and David Charbonneau (arXiv) ApJ in press

This is a very nice analysis of Kepler data looking at the occurrence rate of small (Earth size-ish) planets, and hence a statistical analysis of how many such planets we might expect to find around nearby stars.
The data is restricted to short orbital period planets around low mass stars, because that is where the observations are currently sensitive.
The extrapolations of the data to sun like stars are interesting to say the least, and will be constrained by actual data in the very near future.

The press release quoted an occurrence rate in the Habitable Zone of 6% which is impressive.
The paper has since been modified to predict a higher occurrence rate of 15%.

The change in the prediction came from Ravi Kopparapu, who noticed a discrepancy in the original data which implied a higher occurrence rate.

Ravi, in turn, had a paper out at about the same time: "Habitable Zones Around Main-Sequence Stars: New Estimates" by Kopparapu et al (arXiv) ApJ in press - which updated the classic habitable zone calculations of Kasting and collaborators, based on improved atmospheric opacities and stellar spectral energy distributions.
The new habitable zone estimates, which are conservative underestimates because they do not include all known feedback effects that can stabilize habitable zones, are broader than the previous estimates.

Which leads to Ravi's current paper.

Taking Dressing and Chardonneau's methodology, with the revised occurrence estimate, and applying it to the new improved habitable zone calculations, the occurrence rate of Earth size planets in the habitable zones of low mass stars is about 50%!

ie if we look at the 10 or so known low mass stars within about 10 light years of the Sun, we expect essentially all of them to have low mass planets, and there should be about 5 roughly Earth size planets within the habitable zone of their parent star.
Note that this includes the possibility that some stars may have 2 or 3 small planets within its habitable zone.

This is a fabulous result, and has huge implications for the possibilities of life in the universe, and observing strategies for targeting biosignatures on exoplanets in the near future.

The occurrence numbers are converging on this relatively high fraction, and becoming consistent when different detection techniques are compared.
Further, both the estimates and the definitions of what is habitable try to stick to the conservative side.
There could be more habitable planets than this estimate.

Again, the extrapolation to the occurrence rate for Earth size planets around Sun like stars is also very interesting.
Interestingly high.

PSU Press Release

Handy-dandy updated habitable zone calculator

catdynamics Tue, 03/12/2013 - 04:31

To Do:

catdynamics — Tue, 16 Oct 2012 22:17:18 +0000

To Do:

Given η_Earth=1

Find more terrestrial exoplanets.
Find habitable terrestrial exoplanets.
Find inhabited terrestrial exoplanets.
Go visit.

Consider the following snippets:

Kepler discover 5 planets orbiting inside 0.1 AU
Kepler: A Transiting Circumbinary Planet in a Quadruple Star System
Terrestrial mass planet discovered around α Cen B
Kepler exoplanet survey jeopardized

Kepler has lost one of its reaction wheels.
That was the spare, one more goes, and it will not complete its mission, it will not be able to stay pointed at its target.
Because of stellar jitter, the science goal for Kepler, to quantify η_Earth the fraction of stars with terrestrial planets, will take the full extended mission, not the optimistic nominal mission.

We can not stop now

The discovery of α Cen B b really tells us all we need to know, terrestrial planets are common as dirt, as they say, and it is not just a matter of having found one and packing it in - we now want to get statistical samples studying their distribution of properties, and the individual properties of each unique exoplanet.

Discovering life on other planets is a science goal as important as understanding the nature of dark energy or dark matter, as timely, and as doable with our current and near future capabilities.

"Hammer while the iron is hot" - we need significant observational followup: high cadence searcher like MINERVA; high precision instruments like Habitable Zone Planet Finder, with new improved Laser Combs, and lots of giant telescopes on the ground and in space, finding and following up the wonders out there.

It is timely, it is doable, and it is cheap by standards of modern science.

We can not not do this.

Exoplanets were marginalized by the decadal survey, but, the survey has been overtaken by events, budgetary and scientific. Its identified priorities will not be done, and medium term decisions on research portfolios ought not be based on it.

It is a shame, much sunk effort must be written off, but then that is also what happened with the previous decadal survey, whose priorities were overturned by the mid-decade "Beyond Einstein", leading to most of the identified priorities of that survey never happening.

What should we do?

We should do the next Explorer class mission: preferably both TESS - the transit monitor of the brightest few hundred thousand stars, and FINESSE the spectroscopic characterization of exoplanet atmospheres.

The Next Generation Space Telescope should be finished and flown, if we can.

We should restart the Planet Finder spacecraft, but not making the mistake of going for a grand slam homerun of a megamission on first try.

Start with an engineering coronographic mission, a small one, and a quick one, to look for nearby planets around close bright stars.
Starting with Alpha Centauri!
The odds are good, we should play them.

Then ramp up with Jupiter Imagers and then Terrestrial Planet Finders, and stagger both coronographs and interferometric detectors.

The NSF ought to try to get funding to also strongly ramp up exoplanet studies.
We are short of telescope time, we should not be closing telescopes but going to intense monitoring programs and build new instruments - several of them exploring different technologies, not the current model of picking one desperate lotto winner every few years.

I'm picking on the US because I am worried about its direction in science, the Europeans and Australians are holding steady for now, more funding there ought also be imperative but at least they are progressing coherently.
Asia is also playing impressive catchup and may yet dominate this field.

This is a turning point in humanities existence, as a culture, as an intelligent technological species.

We now know.

We can't ignore this, or get around to it maybe sometime later.
If we have any curiosity, any drive, any ambition left as a species, we have to find out.
Now.

And yes, we should think about going for a visit.
Yes, future technology will hopefully overtake any pathetic crawler we send in α Cen's direction anytime soon, but if we don't develop such technologies by trying now, then we will not have the improved technologies in the future.
That is how we learn to get better at things.

This is our nearest neighbour, it is the next driveway over, we now know they have a building on the lot and we'll soon know if there is a light in the window.
We ought to go visit, say "Hi", it is only neighbourly.

And, you know, bring some baked goods.
Home made bread, or brownies.
Everyone likes home made bread and brownies.

catdynamics Tue, 10/16/2012 - 18:17

Hello neighbour

catdynamics — Tue, 16 Oct 2012 21:36:45 +0000

Hello neighbour

α Centauri B, a mere 4 lightyears away has a terrestrial planet orbiting it.

α Cen B b

The most interesting aspect of the discovery may be the inferences we can make rather than the planet itself.

The discovery by the Geneva Observatory team using the

HARPS spectrograph is a wonderful example of precision high cadence spectroscopy and the ability of observers to find planets wherever they may be. The precision of the measurment is 0.5 m/sec, which is astonishing.

The discovery will be published in Nature (X. Dumusque et al. Nature 17 Oct 2012) thursday, and was due to be announced wednesday at 1 pm, but some idiot broke the embargo and ran the story early, so it is now coming out in pieces.
This particularly annoying, because I teach my exoplanets class at 1:25 wednesday and was planning to surprise them with a change of topic... bloomin' journalists!

The planet has a mass of at least 1.1 Earth masses, with the true mass being 1.1/sin(i) where i is the unknown inclination to the line of sight. It could plausible be as high as 6 earth masses, but is more likely to be in the 1-2 Earth mass range.

Its orbital period is 3.2 days, around the K1V secondary star of the Alpha Centauri system, the primary, α Cen A, being a G star slightly more massive than the Sun. The third star, Proxima Centauri, is a low mass M5 dwarf about 15,000 AU from the inner binary.

The system is a first magnitude star, the closest to the Sun, visible in the southern hemisphere.

At 0.04 AU circular orbital radius the planet has a mean temperature of about 1,200-1,500 K, and is almost certainly tidally locked, with one hemisphere facing the star.

The system is a triple, with α Cen B orbiting the primary with a semi-major axis of about 17 AU, orbiting every 80 years or so, and eccentricity of about 0.5

α Cen A is cannonballing to within 8 AU or α Cen B every century or so.
At closest approach the stars come as close as Saturn is from the Sun, and then recede to almost the distance Neptune is from the Sun.

That doesn't leave a lot of room for planets.
It is very impressive that there is one hiding in there.

Alpha Centauri has been a priority target for the HARPS team, and the competition from Debra Fischer at Yale has been doing high precision, high cadence observations of the system for several years, and we knew already that there were most certainly no giant planets or large ice giants like Neptune orbiting anywhere in the system.

The interesting thing is that the presence of the one planet cuddled up against its parent star is almost certainly the signpost for another planet or two further out, and since we haven't seen them yet, they must have comparable masses of Earth mass or few.

We don't think terrestrial planets can form this close to their parent stars. So something moved the planet from its birth location further out.
It is possible α Cen A pushed it in, through the Kozai Mechanism, but only if the planet's orbit is highly inclined to that of the star, which is somewhat unlikely. It is more likely the orbits are near coplanar (which incidentally implies the planets true mass is near its observed minimum mass).

It take a planet to move a planet.
As a toy scenario, imagine the planet formed a respectable 0.2 AU from α Cen B. A kick up to an eccentricity of 0.9 or so would lead to the planet circularizing to its current orbit.
But, to provide the kick there must have been at least one other comparable mass planet in the system. Two others would be better, at orbital radii in the range 0.25 - 2 AU or so.
It is not unlikely the other planet or planets are still there. They could have been kicked out far enough for α Cen A to knock them out of the system, but that is not very likely.
So there may well be a 1-3 Earth mass planet with a semi-major axis of about 1 AU, and eccentricity of maybe 0.8 or so in the system.
Or two planets, with moderate eccentricities.

Further, if there was a protoplanetary disk around α Cen B which lasted long enough to make terrestrial planets, it seems very likely there was also a disk around α Cen A, and it too must have made terrestrial planets, safely inside 2 AU, but maybe none kicked in as close as the one we just detected.

There are almost certainly several more planets to be discovered there, around both α Cen B and α Cen A!
In principle they may include Earth mass planets orbiting close to the habitable zone of either planet, though at least for α Cen B any planet there is likely to in an uncomfortably eccentric orbit.

ESO Official Press Release - pretty pics with bonus cool videos

catdynamics Tue, 10/16/2012 - 17:36