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.

2012 Review

An Earth-mass planet orbiting Alpha Centauri B. Credit:ESO

2012 brought us yet another remarkable year of extrasolar planet science. While the planet catch for 2012 was a little less than last year’s, the quality and importance of planets revealed this year was amazing. By far the most major results have been the discovery of an ~Earth-mass planetary companion orbiting the secondary component of the nearest star system to our own, Alpha Centauri (see here), and evidence for a system of planets around the nearby star Tau Ceti (see here). I hesitate to draw conclusions from a small amount of data, but the discovery of a terrestrial planet at none other than our nearest neighbour seems to really emphasize the point that terrestrial planets are likely as common as dirt.

A nice system of planets was reported at Gliese 676A consisting of super-Earths and Jovian planets, HATnet and SuperWASP produced more hot Jupiters, and interestingly, a couple sub-Earths may have been found around the nearby star Gliese 436. Spitzer provided us with the first detection of thermal radiation from a super-Earth (see here). A pair of M giants also became the first known to have planets, with planets reported around HD 208527 and HD 220074.

Circumbinary planets were announced around RR Cae, NSVS 14256825, Kepler-34 and Kepler-35 and Kepler-38, which is notable as the first Neptune-sized circumbinary planet.

Kepler results picked up en masse this year. At first it started out nice and slow, with small groups of planets being announced in batches (See here, here, here and here), followed by dozens and dozens of planets.

Interesting Kepler results included Kepler-64, the first quadruple-star system with a planet. The planet is a circumbinary planet, no less. But easily the most important circumbinary planet find was Kepler-47, the first transiting multi-planet circumbinary system. Multi-planet circumbinary systems have been found before but this is the first to have multiple planets transiting. This allows not only for their existence to be much more certain (non-transiting circumbinary planets still suffer from the mass-inclination degeneracy), but allows us to test for coplanarity. The Kepler-47 system demonstrates conclusively that short-period binary stars can host full systems of planets. Another pair of planets with very close orbits to each other, yet very dissimilar densities were reported at Kepler-36. The orbits of the planets in the Kepler-30 system were shown to be well-aligned with their host star’s equator, showing us that systems of planets are, like ours, often neatly arranged and not chaotically scattered.

Good news and bad news about the Kepler spacecraft. The good news is that the mission is extended for another three years. The bad news is that unfortunately, a reaction wheel on the Kepler spacecraft failed, and the mission’s continued usefulness now rests on all of the other reaction wheels remaining operational.

Kepler also unveiled a system of three sub-Earth planets huddled around a dim red dwarf, Kepler-42, which is very similar to Barnard’s Star, as well as a possible small terrestrial planet being evaporated away due to the heat from its star (see here). One of these three planets is Mars-sized(!).

We gained more evidence that the Galaxy is just drowning in planets both from continued Kepler results, HARPS results, and from gravitational microlensing data. Kepler showed us that hot Jupiter systems are frequently lacking in additional planets.

Last but not least, habitable planet candidates were reported around Gliese 163 and HD 40307, with unconfirmed habitable planet candidates reported at Tau Ceti and Gliese 667 C – with two more planets possibly occupying the star’s habitable zone. If GJ 667 Ce is confirmed, then it would be the most promising habitable zone candidate to date, based on its low mass.

At the end of 2011, I gave some wild guesses as to how the extrasolar planet landscape would look like at the end of 2012. Here we are and how have those predictions held up?

The Extrasolar Planets Encyclopaedia lists 854 planets as of the time of this writing, however it is missing quite a few. My own count has us at 899 planets.

• The discovery of a ring system around a transiting planet

There are hints of ring systems (or perhaps rather circumplanetary disk systems) around Fomalhaut b, β Pictoris b, and 1SWASP J140747.93-394542.6 b (see here) but none of these are confirmed. So I’m calling it a missed prediction.

• More low-mass planets in the habitable zone from both radial velocity and transit

Two new habitable planet candidates from radial velocity, none from transit.

• Confirmation of obvious extrasolar planet atmospheric variability (cloud rotations, etc).

I was counting on continued monitoring of the HR 8799 planets to search for atmospheric variability, but it simply didn’t happen (or rather, if it did happen, the results are still pending). So I’m calling this a miss.

2013 could be a very interesting year, especially for Kepler. It seems we are on the verge of finding a true Earth analogue. The detection rate of candidate habitable planets is picking up and we’re really starting to get a list of targets to follow-up in the next decade. Here’s some more brave guesses for the end of 2013:

• 1200 Confirmed planets and planet candidates
• A satellite of an extrasolar planet (an “exomoon”)
• A confirmed ring system around an extrasolar planet
• Phase curve mapping of a sub-Jovian planet