Tag Archives: exoplanet

Kepler’s latest haul

The Kepler mission continues to astound. In May 2013 it suffered a malfunction that ended its primary mission: the spacecraft needs three reaction wheels in order to point its telescope with accuracy, and when the second of four such wheels failed the mission was over. However, a team of scientists and engineers developed a method for using the pressure of sunlight as a third “virtual” reaction wheel: the method works (albeit with reduced capabilities and with the need for a prodigious amount of signal analysis). Kepler is back in business.

In December 2014, astronomers announced that Kepler had spotted a new planet (HIP 116454b), a discovery that was subsequently confirmed by other telescopes. HIP 116454b is a super-Earth: it has about 12 times the mass of Earth and is about 2.5 times larger. It orbits close to a K-type orange dwarf. (The Allen Telescope Array has observed this planet, looking for signals in the range 1000 to 2250 MHz.  It didn’t hear anything.)

And further analysis of existing pre-malfunction Kepler data has taken its tally of exoplanet discoveries past 1000. The latest batch includes the most Earth-like planet found to date: Kepler 438b is just 12% larger than Earth.

Kepler continues to produce surprises!




Almost Lagash

The accolade for “the greatest science-fiction story ever written” usually goes to Isaac Asimov’s wonderful tale Nightfall (though personally I’d agree with the master himself, and say that The Last Question is a better story). If you haven’t read Nightfall, it’s a story set on the planet Lagash. The planet is unusual in that it possesses a stable orbit around six suns. Inhabitants of Lagash have evolved in an environment in which they never see night, never see darkness. The story hinges on what happens during an eclipse when, for the first time, the natives of Lagash experience nightfall. Read the story. You’ll love it.

Illustration of Nightfall

Asimov’s “Nightfall”
(Credit: Dimension X)

Critics have often pointed out a weakness in the story: a stable orbit around six stars is, they say, impossible. Well, this week a team of astronomers working from Kepler data have posted a paper on arXiv (“Planet Hunters: A Transiting Circumbinary Planet in a Quadruple Star System“) that tells of the discovery of a planet that’s being pulled by the gravitational tug of four planets. This isn’t quite Lagash, but it’s a planet that possesses an apparently stable orbit in a very complicated environment. Planetary systems are clearly more complicated than we thought. Maybe a planet like Lagash isn’t impossible after all.

The most habitable exoplanet?

Pressures of work mean that I’m way behind on my reading. I’ve just got round to reading the December 2011 issue of Astrobiology. One interesting article in this issue is A Two-Tiered Approach to Assessing the Habitability of Exoplanets, by Dirk Schulze-Makuch et al.

The authors suggest the use of a two-tiered classification scheme in order to assess exoplanet habitability. The Earth Similarity Index (ESI), as its name implies, ranks planets based on their similarity to Earth in terms of mass, size, temperature and so on. The Planetary Habitability Index (PHI) ranks planets according to the presence of a stable substrate for life, available energy, appropriate chemistry, and the potential for the planet to hold a liquid solvent. The authors have formulated both indices in such a way that they can be updated as our knowledge advances; this is particularly important for the second tier of the classification scheme, the PHI, since that index requires more information than currently exists for any exoplanet.

The fun bit of the paper, though, is the appearance of a “top-10” list of objects as given by the ESI and the PHI.

The planet with the highest Earth Similarity Index is, of course, Earth. More interestingly, the object with the second-highest Earth Similarity Index is Gliese 581g (Earth has an ESI of 1; Gliese 581g has an ESI of 0.89).

The planet with the highest Planetary Habitability Index is, again no surprise, Earth (which has a PHI of 0.96). Titan, Mars and Europa occupy places 2-4 on the list. The exoplanet with the highest Planetary Habitability Index is, once again, Gliese 581g (with a PHI of 0.45).

So – is Gliese 581g the best place to be looking for alien life (perhaps, as has been suggested, by analysing reflected light from the planet in a search for biomarkers such as the presence of chlorophyll). Maybe. But it’s worth pointing out that it’s not at all certain that Gliese 581g even exists! The Lick-Carnegie Exoplanet Survey ‘discovered’ this exoplanet in September 2010; but the planet did not show up in an analysis of data from the High Accuracy Radial Velocity Planet Searcher. As things stand today, the existence of the planet is unconfirmed.

Perhaps the most habitable exoplanet, as of today, will turn out to have been a mirage.


A team of astronomers have recently published results of observations made by Hubble of the exoplanet GJ 1214b. They didn’t discover the planet: that honour belongs to the MEarth project, which uses robotic telescopes to survey nearby M dwarf stars in search of new Earth-like exoplanets. This project spotted GJ 1214b back in 2009. What’s new is that a Hubble was used to observer the planet during transit: because the star’s light is filtered through the planet’s atmosphere, astronomers can infer what gases might be present. The best fit to the Hubble data is that GJ 1214b has a dense atmosphere of water vapour.

Various observations allow astronomers to pin down the planet’s mass, size and orbital parameters. It turns out that GJ 1214b has a radius 2.7 times that of Earth’s and a mass 6.5 times that of Earth’s, which means its average density is about twice that of water. In comparison, Earth’s average density is 5.5 times that of water. In other words, GJ 1214b holds much more water than does Earth. It’s a true waterworld.

Artist's impression of the exoplanet GJ 1214b

Artist's impression of the exoplanet GJ 1214b
Credit: NASA, ESA, and D. Aguilar

Water … so does that mean GJ 1214b could be home to life? Well that’s unlikely, because the planet orbits just two million kilometres away from its red-dwarf star. It’s temperature will be about 230°C. However, theories of planet formation suggest that GJ 1214b will have formed at a large distance from its star and then subsequently migrated to its current position. It therefore must have passed through the habitable zone.

There might not be life there now, but it’s possible – just possible – that life may have been there once.

Earths are common, not rare

The paper One or more bound planets per Milky Way star from microlensing observations, which appears in today’s Nature, settles the question of whether planetary systems are common.

The paper’s 42 authors (who constitute a truly global collaboration of astronomers) analysed gravitational microlensing data gathered in the period 2002 to 2007. Now, gravitational microlensing – the short-term brightening of a star that occurs when the star, the planet of an intervening star and one of our telescopes move into alignment – is rare: at any particular time, less than one star in a million will be subject to a microlensing event. So although gravitational microlensing is a well-established planetary detection technique, it is less productive than either the radial velocity method (which is used by experiments such as HARPS) or the transit photometry method (which is being used by the phenomenally successful Kepler Mission). Thus the present paper does not announce the sudden discovery of a large number of previously unknown exoplanets. Rather, by statistically analysing the number of events that were detected, the team was able to estimate how many exoplanets are likely to exist out there.

The bottom line is: stars are more likely than not to possess planets. Our Galaxy must be teeming with planets.

Drake equation

The Drake equation
Credit: Waifer X (Flickr CC)

The Drake equation contains a term fp – the fraction of stars formed that will have planets. Once upon a time, not that long ago, some astronomers believed that planets were rare. Perhaps, they argued, fp was small. Now we know for certain that isn’t the case. We know for certain that we can’t look at this term as a solution to the Fermi paradox.

It seems to me that every astronomical advance is simply sharpening the paradox.