Imagining and Planning Interstellar Exploration

We’ve looked at a couple of exoplanet issues this week that bear further comment. The first is that different detection methods can be usefully combined to cover different scenarios. If radial velocity works best with larger planets closer to their star, direct imaging takes us deep into the outer planetary system. We saw yesterday how both imaging and radial velocity could be used to probe subgiant stars. We routinely use RV as a check on transiting planet candidates. And gravitational microlensing can find planets at a wide range of separation from their primary.

I think microlensing has plenty to teach us, though I’m sensitive to the criticism voiced in comments here that we’re dealing with non-repeating events when we have a microlensing detection. Centauri Dreams reader coolstar has also noted that distance may be a factor, questioning whether some of the resources by way of telescope hardware that we’re putting into microlensing studies wouldn’t be better employed looking at nearby worlds. After all, a new paper by Timothy Morton and Jonathan Swift makes the case that small planets around M-dwarfs are even more plentiful than recent studies reported here have shown. Maybe we should be putting more of an effort into finding these through transit studies?

If different detection methods each have their uses, radically different classes of telescope can likewise probe for planets. John Johnson (Caltech) heads up Project Minerva, a telescope array dedicated to Earth-like planets around nearby stars. The plan is to use small-aperture robotic telescopes atop Palomar Mountain to perform photometry and high-resolution spectroscopy, making it, according to Caltech’s site on the project, “the first U.S. observatory dedicated to exoplanetary science capable of both precise radial velocimetry and transit studies.” Following an initial design review, the project has now seen delivery of the enclosure for its telescopes.

Image: Minerva’s hardware. Minerva will be an array of small-aperture robotic telescopes to be built atop Palomar Mountain outfitted for both photometry and high-resolution spectroscopy. It will be the first U.S. observatory dedicated to exoplanetary science capable of both precise radial velocimetry and transit studies. The multi-telescope concept will be implemented to either observe separate targets or a single target with a larger effective aperture. The flexibility of the observatory will maximize scientific potential and also provide ample opportunities for education and public outreach. The design and implementation of MINERVA will be carried out by postdoctoral and student researchers at Caltech. Credit: Caltech/John Johnson.

TESS, the Transiting Exoplanet Survey Satellite, stands as a logical follow-on to the Kepler mission because it takes the hunt to nearby stars even as Kepler’s candidate totals push on toward 3000. But as we wait for new instruments to come online, including the upcoming James Webb Space Telescope and ground-based systems like the European Extremely Large Telescope, it’s great to see grad students like Johnson’s Exolab crew coming up with solutions like Minerva. It’s an inexpensive effort that could gain real traction, and as Johnson told one reporter, “If we lived in an ideal world, we wouldn’t do Minerva because we’d have money from our funding agencies.” But then, who ever said it was an ideal world?

Minerva had a poster devoted to it at the most recent meeting of the American Astronomical Society. The plan is to use an array of four instruments, each with a 0.7-meter mirror, that will over a period of three years scan stars within about 75 light years of the Earth. To get a look at it, see Unblinking Baby Telescopes Will Hunt Exoplanets for Cheap, a recent piece that explains the project this way:

The Minerva telescopes will look for a slight wobble that indicates a planet is tugging gravitationally on those stars. Using this data, [graduate student Kristina] Hogstrom said that her team expects to find around a dozen new local planets, most of them two to three times the size of Earth and a few of them orbiting in the habitable zone where liquid water could exist. The project will also find Earth-sized planets and perhaps smaller, but these will likely orbit too close to their parent star to host life.

What I like about this, and I can see why Centauri Dreams readers find it interesting, is that it’s a dedicated exoplanet project that, Kepler-like, stares at a designated number of stars. The article quotes Johnson as saying that astronomers aren’t really in planet ‘hunting’ mode these days so much as engaged in routine planet ‘gathering,’ which tells you how far we’ve come in this remarkable enterprise in the few short years since 1995, when the announcement of the first planet discovered orbiting a main sequence star was made by Michel Mayor and Didier Queloz, to be quickly confirmed by Geoff Marcy and Paul Butler.

So here we are with off-the-shelf hardware and a custom-built spectrometer, running up a tab of about $3.5 million as compared to the $600 million that the relatively cheap space-based Kepler cost. We’re probably less than a year away from the datastream beginning to flow at the Minerva site. Modest funding and limited resources used imaginatively in arrays of small dedicated telescopes may be forced upon us by circumstance, but we’re learning that good science can flow from such projects and private institutions can find ways to fund them.

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Our exoplanet detection methods have their limits. Radial velocity studies work great in the inner regions of planetary systems, but become more challenging as we move away from the star. Direct imaging is the reverse — we’re most likely to see a distant planet if it’s both large and well separated from the primary. Clearly we need to take the best data from each available method to characterize a planetary system. But direct images are rare and some stars — A-class in particular — are tricky for RV studies because of jitter and other problems. If you want to get in close to an intermediate mass star to look for planets or a debris disk, the way to do it seems to be to study ‘retired’ stars sitting on the subgiant branch of the Hertzsprung-Russell diagram.

These are stars that have slowed or stopped fusing hydrogen in their cores. Core contraction raises the star’s temperature enough to fuse hydrogen in a shell surrounding the core and the star begins to swell up toward giant status. A team led by Amy Bonsor (Institute de Planétologie et d’Astrophysique de Grenoble) has been looking at κ Coronae Borealis (κ CrB), also known as HD 142091, using data from the Herschel space telescope in the far infrared. κ CrB turns out to be unique, offering an example of a debris disc around a subgiant and “…a rare example of an intermediate mass star, where both planets and planetesimal belts have been detected.”

That brief quote from the paper on this work goes on to note that the planets and debris disk here can tell us something about planet formation around such stars. There is much we need to learn — some studies have found that more giant planets tend to form around higher mass stars than lower ones, and we’d like to tune up planet formation models in this area. κ Coronae Borealis is a star of about 1.8 solar masses at a distance of 100 light years, some 2.5 billion years old. Previous radial velocity work has revealed one giant planet of about two Jupiter masses orbiting at a distance similar to the asteroid belt in our Solar System. The paper presents Keck measurements offering evidence for a second companion whose mass is unclear.

Image: Kappa Coronae Borealis, based on Herschel PACS observations at 100 μm. North is up and east is left. The star is in the centre of the frame (not visible in this graphic) with an excess of infrared emission detected around it, interpreted as a dusty debris disc containing asteroids and/or comets. The inclination of the planetary system is constrained at an angle of 60º from face-on. Credit: ESA/Bonsor et al (2013).

These observations yield several possible configurations for the debris disk and planets, with the interesting possibility that the disk may be divided into two narrow belts (centered at 40 AU and 165 AU respectively) with the outermost planet being in actuality a brown dwarf. Another model has a single continuous dust belt extending from 20 to 220 AU, with a planet sculpting the inner edge of the disk. A third possibility: The disk is being stirred by two planets so that the rate of dust production in the disk peaks at roughly 80 AU from the star. Says Bonsor: “It is a mysterious and intriguing system: is there a planet or even two planets sculpting one wide disc, or does the star have a brown dwarf companion that has split the disc in two?”

From the paper:

As the first example of a planetary system orbiting a subgiant, a more detailed population study is required to determine whether or not κ CrB is unusual, nonetheless, this work suggests that κ CrB did not suffer any dynamical instability that cleared out its planetary system, similar to the Late Heavy Bombardment. As the first example of a > 1.4M⊙ star, with a giant planet interior to 8AU, where there is also resolved imaging of a debris disc, κ CrB provides a good example system from which to further our understanding of planetary systems around intermediate mass stars.

The paper is Bonsor et al., “Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant κ CrB,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint). More in this ESA news release.

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The news that NASA has approved the TESS mission kept my mood elevated all weekend. TESS (Transiting Exoplanet Survey Satellite) has been the logical NASA follow-up to Kepler ever since the Space Interferometry Mission was canceled in 2010. The point is that Kepler looks at a field of stars with the goal of developing a statistical analysis, helping us (ultimately) to home in on the value for ηEarth (Eta_Earth), the fraction of stars orbited by planets like the Earth.

To do this, Kepler is looking out along the Orion Arm of the galaxy, with almost all the stars in its field of view between 600 and 3000 light years away. In fact, fewer than one percent of Kepler’s 156,000 stars are closer than 600 light years. There are plenty of stars beyond 3000 light years, but as we push beyond this distance, the stars become too faint for Kepler’s transit methods to be effective. The carefully chosen field in Cygnus and Lyra is ideal for Kepler’s statistical data but the next question to ask is how many Earth-like planets are to be found around relatively nearby stars.

TESS is led by principal investigator George Ricker (MIT Kavli Institute for Astrophysics and Space Research), whose team will be working with an array of wide-field cameras to perform an all-sky survey, as opposed to the intense ‘stare’ Kepler makes at a particular starfield. Ricker says the mission should be able to identify thousands of new planets in the solar neighborhood in a survey that will cover 400 times as much sky as any previous exoplanet mission.

We all have ideal missions we’d like to see fly, from the Space Interferometry Mission to some sort of grandly designed Terrestrial Planet Finder, but among the realistic options in front of us, TESS makes abundant sense. It will home in on small rocky planets and help us measure their mass, density, size and orbit, along with offering up data on their atmospheres. And you can also think of TESS as something of a pointer scope for the James Webb Space Telescope and other future instruments that will begin the hunt for astrobiological signatures on other worlds.

Image: TESS’s primary goal would be to identify terrestrial planets orbiting nearby stars. Credit: MIT Kavli Institute for Astrophysics & Space Research.

“The TESS legacy,” says Ricker, “will be a catalog of the nearest and brightest main-sequence stars hosting transiting exoplanets, which will forever be the most favorable targets for detailed investigations.” That makes the $200 million funding for this bird money well spent. In an interview on the Kavli Foundation site, Ricker adds that the interactions of multiple planets as they orbit their host star, spotted through transit timing variations, can aid the search:

Measuring transit time variations is particularly important because if you find a large Neptune- or Saturn-sized planet that appears to be tugged a little bit, you can make an estimate of how massive the planet is that is actually doing the tugging. Measuring transit time variations, which is a technique enabled by Kepler data, is a bit like successively unnesting a Russian matryoshka doll – you go a few layers down and you can find smaller and smaller planets in the system. That’s one of the things that doing transit time variations, during the TESS mission, will enable us to do.

In other words, we’ll have TESS spotting planets around stars bright enough for ground-based telescopes to home in on these transit time variations (TTV), something we can’t readily do with most of the Kepler planets because their stars are too faint. The system is designed to detect changes in the intensity of a star’s light down to 40 parts per million. By comparison, Earth viewed from outside the Solar System would cause a transit drop of about 85 parts per million. Ricker estimates that TESS will be able to detect as many as 2700 planets, including several hundred Earth-size worlds. The TESS mission is now scheduled for launch in 2017.

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While transit and radial velocity methods get most of the press when it comes to finding exoplanets, gravitational microlensing offers an independent alternative. Here a star passes in front of a far more distant object, causing the light from the source to be gravitationally ‘bent’ by the intervening star. The useful thing for exoplanet work is that if the ‘lensing’ star is orbited by one or more planets, they can leave their own signature in the microlensing event. And indeed, microlensing collaborations like MOA (Microlensing Observations in Astrophysics) and OGLE (Optical Gravitational Lensing Experiment) have made the method pay off in exoplanet discoveries.

Image: Gravitational microlensing relies on chance line-ups between an intervening star with planetary system and a more distant light source. Credit: California Institute of Technology.

Now researchers at the University of Auckland are proposing to measure low-mass planets, planets as small as the Earth, using these methods. Microlensing events vary in terms of the degree of magnification they provide, with both MOA and OGLE finding some events with extremely high magnifications. You would think the higher the magnification, the better, but this turns out to be not necessarily the case, as the university’s Phil Yock and colleagues explain in a new paper. Their simulations clarify the methods we can use to detect Earth-sized planets:

Our conclusion is that low-mass planets can be efficiently searched for in events with moderately high magnifications of order 50-200, and that the greater frequency of these events in comparison to those with higher magnifications lends advantage to monitoring them. However, to take full advantage of their discovery potential, telescopes with apertures in the range 1-2m would be needed to photometer them with sufficient precision to detect low-mass planets.

A network like this is, in fact, being deployed by the Las Cumbres Observatory Global Telescope Network, working in collaboration with SUPA/St Andrews (Scottish Universities Physics Alliance). These are 1- and 2-meter instruments, with three telescopes each in Chile, Australia and South Africa, and one each in Hawaii and Texas. Working with a telescope in the Canary Islands and supplementing its data with several other southern hemisphere instruments, the network, according to Yock and colleagues, ought to be able to help us not only detect Earth-sized planets but measure them at substantial distances from their star. Says Yock:

“Our proposal is to measure the number of Earth-mass planets orbiting stars at distances typically twice the Sun-Earth distance. Our planets will therefore be cooler than the Earth. By interpolating between the Kepler and MOA results, we should get a good estimate of the number of Earth-like, habitable planets in the Galaxy. We anticipate a number in the order of 100 billion.”

It’s an intriguing idea. The Kepler effort is all about developing a statistical read on how often Earth-class planets occur in the galaxy by looking at a huge number of stars. Yock’s work demonstrates that low-mass planets can come out of microlensing observations through continuous network monitoring. The paper notes that in recent times, about 10 events with magnifications over 200 have been detected by MOA and OGLE every year. Dropping the magnification requirement to 50 should quadruple the detection rate, allowing us to develop an independent check on measurements made by radial velocity and transit studies.

Consider how useful microlensing can be. We are not burdened, as with radial velocity and transits, with a measurement more sensitive to larger worlds closer to their host stars. The planets thus far detected through microlensing have had masses ranging from a few Earth masses to a few Jupiter masses, while their separation from their hosts has been on the order of several AU. This provides, says the paper, ‘a representative sample of the distribution of planets between us and the centre of the Galaxy that orbit a relatively unbiased sample of host stars.’

Thus we can supplement other ways of estimating the abundance of cool planets in the Milky Way by developing a statistically significant sample of detections through the kind of microlensing events Yock describes. The paper is Abe et al., “Extending the Planetary Mass Function to Earth Mass by Microlensing at Moderately High Magnification,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint).

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