## 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.

## 2011 Review

Arguably the most important discovery of 2011: Earth-sized exoplanets

2011 was a banner year for extrasolar planet science with the Kepler results really beginning to come in. Among the more interesting:

Kepler results:

• Kepler-10: Kepler’s first rocky super-Earth, with a transiting Neptune further out.
• Kepler-11: A system of six transiting super-Earths with anomalously low density.
• Kepler-14: A massive hot Jupiter in a binary system.
• Kepler-16: The first transiting circumbinary planet around an eclipsing binary star.
• Kepler-18: A system of three planets: A super-Earth and two inflated Neptunes in a 2:1 resonance. Very similar to Kepler-9 but scaled down in masses.
• Kepler-19: A transiting sub-Jovian planet and the first case of the discovery of a second planet through transit timing variations in the transiting planet.
• Kepler-20: A system of five planets, two of which are ~Earth-sized.
• Kepler-21: A transiting super-Earth around a bright (V = 8.27) star.
• Kepler-22: A transiting “mini-Neptune” in the habitable zone, and the first transiting planet in the habitable zone of any star.
• KOI-423: First transiting planet around a subgiant star.
• KOI-730: A remarkable system of four planets in a 1:2:4:8 resonance.
• KOI-55: What appears to be two remnant cores of gas giants engulfed by their parent star during its red giant phase.
HARPS results:

• 82 Eri: Three low mass planets only a few times the mass of Earth.
• HD 136352: Three super-Earth/sub-Jovian planets.
• HD 39194: Three super-Earths.
• HD 134606: Three Neptunes.
• HD 215152: Two lower-mass super-Earths.
• Gliese 667 C: A second planet of a few Earth-masses in the habitable zone.
• HD 85512: A super-Earth on the inner edge of the habitable zone.

On the orbital dynamics front, in January, it was found that the HD 37124 system, which hosts three intermediate period gas giant planets of roughly equal mass, may have a 2:1 resonance for the orbit of two of its planets. Furthermore, the planet candidate orbiting Rho Coronae Borealis, one of the first planet candidates, was proven to have a true mass far outside the planetary regime. The recovery of the planets of HR 8799 in old HST data has permitted the architecture of the system to be much more constrained.

A planet around a naked-eye giant star Alpha Arietis was reported in April. Later, in August, it was revealed that a new planetary mass object has been found orbiting a pulsar. On the subject of post-main sequence stars, a candidate planet was imaged orbiting a white dwarf. On the other side of the main sequence, a planet had been found in the late stages of formation at LkCa 15.

Gravitational microlensing provided us some constraints on the abundance of rogue giant planets as well as another cold super-Earth. It turns out that rogue giant planets may be twice as frequent as main sequence stars.

Several planets around eclipsing binaries were found this year, including planets at UZ For, HU Aqr, Kepler-16, and NY Vir.

Without a doubt, one of the most exciting stories of 2011 is the discovery that one of the first known super-Earths, the innermost planet at 55 Cancri, transits its star. This is the brightest star known to have a transiting planet, and it will prove very useful for the study of these kinds of planets. While it was initially thought that the planet was rocky and iron-rich, later observations suggest that the planet must have a significant envelope of volatiles.

In summary, 2011 has been an astounding year. The focus has shifted away from gas giant planets to sub-Jovian planets — Neptunes, mini-Neptunes and super-Earths. Below is a graph that shows this year’s total planet catch compared to previous years. One thing is clear: Not only is the galaxy full of planets, but we can look forward to seeing a huge number more in the near future.

Now for some fun, some predictions for what we might have by the end of 2012:

• 1,000 Planets on the Extrasolar Planets Encyclopaedia
• The discovery of a ring system around a transiting planet
• More low-mass planets in the habitable zone from both radial velocity and transit
• Confirmation of obvious extrasolar planet atmospheric variability (cloud rotations, etc).