## Revelation

Pluto as seen by New Horizons on 13 July 2015

Tomorrow is a rather big day for the exploration of the Solar System. A world that has long represented our ignorance about the outer solar system will come into splendid view, as revealed by the New Horizons spacecraft. Pluto, a dwarf planet discovered in 1930, has always been that question-mark at the end of the book about the Solar System.

I tend not to cover solar system exploration on this blog because it’s dedicated to extrasolar planet science, but I think there are some interesting parallels one can make between Pluto and extrasolar planets. Until recently, Pluto has just been an unresolved dot in the sky. Indirect methods had allowed for the mapping of crude surface features — we came to learn that Pluto has significant albedo variation across its surface, with patches of bright and dark, but other than that, we knew nothing about their composition or even what caused them. To a large extent, we still don’t. We now know (as of the past couple days) that Pluto’s north pole is covered in an ice cap dominated by nitrogen and methane, and that the dark regions are comparably methane-poor, but we don’t really understand a lot of what’s going on yet. But the advance in our knowledge of Pluto over the past month has been truly revolutionary.

In the not-too-distant future, indirect methods will begin to yield crude albedo maps of extrasolar planets. These maps may have a similar quality to those acquired for Pluto. It will be worth remembering, however, that there’s so much more about those planets that we won’t be able to see simply because we lack the ability to send a probe of some sort to those extreme distances.

New Horizons Map Comparison

The image above shows a comparison between our “best map” of Pluto before New Horizons, and what is currently our limit of knowledge (with full credit to Bjorn Johnson). Quite a difference a spacecraft visit makes! The top map is an average of five separate maps acquired by HST and ground photometry, so an important caveat here is that not only is the wavelength coverage different than New Horizons’ LORRI camera made to use the bottom map, but the mapping technique is different, too, and prone to different biases. That all being said, there is a decent amount of similarity between the two.

We may live to see an exoplanet’s surface resolved to an extent similar to Pluto’s before New Horizons, however it will be several generations before we are able to map an exoplanet with the same level of precision as the bottom map in the image above.

## Meeting the Neighbours

One of six mirrors for the Giant Magellan Telescope

The proximity of a planet to our own solar system will be critically important in the near-future when we begin to characterise the atmospheres and assess the potential habitability of extrasolar planets through direct imaging spectroscopy. Additionally, the nearest extrasolar planets will likely be the first targets of interstellar probes launched by humanity to investigate these worlds up close. To this end, there is considerable interest in discovering planetary companions to nearby sun-like stars. I’ve compiled a brief table below to show the nearest sun-like (FGK) stars to us. I’ve omitted the sea of red dwarfs (as well as a couple A-type stars: Sirius and Altair) interspersed between them in the interests of brievity.

 Nearest Sun-like (FGK) Star Systems D (pc) Spectral Type(s) Sol 0.0 G2V Alpha Centauri 1.3 G2V + K1V + M5V Epsilon Eridani 3.2 K2V Procyon 3.5 F5IV + DQZ 61 Cygni 3.5 K5V + K7V Epsilon Indi 3.6 K5V + T1V + T6V Tau Ceti 3.6 G8V Groombridge 1618 4.9 K7V 40 Eridani 4.9 K1V + DA + M4V

This interest in our sun’s nearest neighbors can hardly be said to be confined to the scientific community. How many of these star names do you recognise, even if simply from works of fiction?

Until this year, there has been evidence for a giant planet around ε Eri (though the existence of this planet has been questioned recently). But over the last couple of months, we have seen extraordinary announcements of planetary companions around both α Cen and τ Cet.

Firstly, and perhaps most importantly, the discovery of a planet around α Cen is an important milestone in our attempts to understand our place in the Universe. The nearest star system to our own has been found to have a planet – only twenty years after it was not known for sure that extrasolar planets existed at all. But for the details (which are likely not news to the reader): The planet orbits the secondary star in the system in a 3 day orbit, exposing the planet to temperatures that completely throw the question of habitability out the window. But we now know that the system was conductive to planet formation billions of years ago, and we have learned that planets often come in groups. There may yet be more planets around α Cen B, and perhaps even planets around α Cen A as well.

But from the perspective of technological progress, the planetary companion reported around $\alpha$Cen B has a minimum mass that is roughly equal to that of Earth. This planet isn’t some super-Earth or a mini-Neptune: It’s almost certainly a terrestrial planet, and it is the lowest-mass planet detected thusfar from Doppler spectroscopy.

With an increasing number of planet candidates in their star’s habitable zones, and an increasing number of planets discovered nearby, we are approaching a time where there will be a known sample of small planets in the habitable zones of the nearest stars. These planets will likely be our best targets for a search for biosignatures in their atmospheres. Large ground-based telescopes with aperture sizes on the order of 25 – 30 metres, such as the Thirty Metre Telescope and the Giant Magellan Telescope will likely be the key to characterising the atmospheres of these planets.

Thirty Metre Telescope (Concept image)

There are about 125 main sequence stars within 8 pc. Depending on what the abundance and properties of planets in general are in the solar neighbourhood, there may well be a hundred or so small planets that will be available to characterisation by the next generation of ground-based telescopes. You can estimate the underlying distribution of planets (with help from the Kepler mission results) and calculate their angular separation from their stars and their brightness contrast. This can give you an idea of what planets exist in our celestial backyard that are available to us for direct study. Looking in the infrared will make this all easier as the contrast is greater between the two (still extreme, but not as difficult as visible light). A study was recently posted to arXiv about this specifically.

It is notable that the recentlty discovered low-mass planet candidate α Cen Bb presents a contrast of 10-7 and is eminently observable in this baseline scenario (assuming a radius of 1.1 RE) … Despite the worse contrast achieved on this fainter star, the massive planets GJ 876 b and c can be directly characterised if Ag (Rp / RE)2 > ~19 and ~6, respectively (Ag > ~0.02) … The currently known planets GJ 139 c and d can also be characterised of Ag(Rp / RE)2 > ~0.9 and ~1.8 respectively (i.e. Ag > ~0.4), and the planet candidates τ Cet b, c, and d could be characterised for Ag (Rp / RE)2 > ~0.08, ~0.4, and ~1.2 respectively (Ag > ~0.04, ~0.14, and ~0.35). Theoretical efforts to model all these planet’ near-infrared reflectance are clearly warranted.

With nearby planets at α Cen, GJ 876, τ Cet and 82 Eri (GJ 139) being potentially available to direct atmospheric characterisation in the next decade, we are approaching an exciting time where we will have some idea of what these planets are like. This understanding will transcend the basic understanding we have been able to gather from transit light curves and radial velocity.

Still, it will be some time until these planets are known as well as the planets of the Solar System. Our understanding of them will likely be comparable to our current understanding of Pluto. We know there are surface features, we know there are ices of various compositions, but beyond that, Pluto is very much a mystery. Unlike Pluto, a world where in three years time we will have close-up images of thanks to the New Horizons spacecraft, we are not likely to see close-up images of extrasolar planets for the foreseeable future, at least certainly not within our lifetime. Our generation, and likely the next several, will be forced to be content with getting only hints of the nature of the surface conditions on planets outside our solar system. But those hints may be very revealing. They can imply the presence of interesting chemistry and perhaps even biological activity. The limited nature of the understanding of these worlds that we will see unfold in our lifetime is therefore no reason to be pessimistic or to despair. There is much to learn even in the near future, and we should be thankful to be alive in an exciting time where we are beginning to discover the nearest planetary systems to our own and, in the near future, characterise their atmospheres.