I was quoted in the newspaper today. One problem with talking to journalists, is that you don’t always know quite how they’re going to represent what you said, or – even – if you’re going to end up having said something silly; you don’t get much warning and you, typically, don’t get a chance to proof read what they end up writing. This article, however, seems fine; I’m not sure if I actually said what I’m quoted as saying, but it seems pretty close to something I would have said.
The article itself is about the recent announcement, by NASA, of 1284 new exoplanets. Just in case anyone doesn’t know, an exoplanet is a planet in orbit around a star other than the Sun. These new exoplanets were discovered by NASA’s Kepler satellite, which uses the transit method. The transit method basically works by staring at as many stars as possible (150000 in the case of the Kepler satellite) and trying to find those that show periodic dips in brightness. This would indicate something passing in front of the star. The relative dip in brightness can then be used to infer the radius of this object, and the period can be used to infer its distance from the star.
One problem with this method is that there can be lots of false positives; there are many things that aren’t planets that can cause what appear to be periodic dips in a star’s brightness. However, the Kepler data is so exquisite that they can rule out many of these false positives. That’s what’s happened here. These new exoplanets were amongst many candidate exoplanets detected a few years ago. The analysis now indicates that these 1284 candidates are almost certainly exoplanets, and hence have been announced as such.
This gives me an opportunity to discuss some of my own research. As the article says, I’m part of the HARPS-N consortium. Although the transit method has been extremely successful, it essentially only allows one to determine the radius of the planet and its distance from the star. If there are multiple planets, one can sometimes infer the planet masses from the timing of the transits, but this doesn’t work for all systems. However, in a planetary system, the star and planets all orbit the common centre of mass. This means that at some times the star will be coming towards us, and at other times away.
Telescopio Nazionale Galileo
HARPS-N is a high-resolution spectrograph, part built in Edinburgh and located on the 3.6m Telescopio Nazionale Galileo
. What it does is measure small shifts in the star’s spectrum which can then be used – via the Doppler effect – to determine the star’s radial (line-of-sight) velocity:
where is the rest-frame wavelength of a specific spectral line, is the shift in this wavelength, is the radial velocity of the star, and is the speed of light. From these small shifts in the spectral lines, you can determine the radial velocity of the star. If the radial velocity of the star shows periodic features, then one can infer that it must have companions (planets) and one can use this to infer the mass of these companions, their distance from their host star, and the eccentricity (or the circularity) of their orbits.
Credit: Motalebi et al. (2015)
The figure on the left shows the radial velocity curves for 3 rocky planets in a 4 planet system that we discovered last year
. I should be clear that the radial velocity is that of the star, and each radial velocity curve has removed the contribution due to all the other planets in the system. The top two curves are quite sinusoidal and indicate that these two planets are on roughly circular orbits. The asymmetry in the bottom curve indicates that this planet’s orbit is somewhat eccentric.
Okay, this post is getting rather long, but we’re getting to the point I was wanting to highlight. If you look at the figure on the left, you’ll note that the radial velocity amplitudes are a few m/s. The spectral resolution of HARPS-N is . This is the inverse of the smallest relative wavelength change that can be measured by the instrument
If you look at the formula for the Doppler shift that I included above, you can relate this to the spectral resolution through
If HARPS-N has and , then . Hmmmm, if this is the smallest radial velocity that we can measure, how can we have measured radial velocities of only a few m/s? The reason is that we measure across a wavelength range (383nm – 690nm) where there are lots and lots and lots of spectral lines, and then we cross-correlate with the known spectrum of the type of star we’re observing. The peak in the cross correlation function then gives the wavelength shift, from which we can determine the radial velocity of the star. You then need to repeat this a number of times (maybe 30 to 60) over the course of a year or so, to then produce the radial velocity curve from which you can determine if there is a companion planet, and – if there is – the properties of that planet.
So, even though we can’t directly determine the shift in individual lines, we can still determine the wavelength shift and – hence – the radial velocity of the host star. Given that we can’t actually see the shifts directly, how can we be confident that what we’ve measured really is indicative of a companion planet? One way is that different teams observe the same system and get the same result. Another is that some of the systems we observe are Kepler targets that are already known to probably host planets. The radial velocity results for those systems are consistent with what is already known from the transit measurements. Finally, and this applies to the 4-planet system I mentioned earlier; some of those detected via the radial velocity measurement are then found to also transit their host stars. Again, the results are consistent.
Mass-radius relations for exoplanets (Motalebi et al. 2015)
Maybe I’ll finish by pointing out another reason why combining radial velocity and transit measurements can be so powerful. The radial velocity measurements give the mass of the planet, while the transit meaurement gives its radius. Together they give the density, from which one can infer the internal compostion. The figure on the right shows the mass-radius relation for a number of known exoplanets, including Kepler-78b (K78b) which our team characterised a few years ago and is still the most similar – in composition and size – to the Earth, and HD219134b, one of those shown in the radial velocity figure above.
What’s clear is that there are a number of known exoplanets with compositions that appear very similar to the Earth. However, to date, these are all planets that are very close to their parent stars and, therefore, are almost certainly far too hot to host life. To date, we do not know of any genuine Earth-like exoplanets, in terms of composition, size, and distance from a star similar to the Sun. This is one reason why I think we have to be careful when talking to journalists about this topic. It’s easy to make them think that we’ve found something Earth-like and, hence, habitable, when really it is simply a rocky planet with a composition similar to that of the Earth, but almost certainly too hot to harbour life. For the moment, I would be very cautious about accepting any claims of having found a habitable, or even potentially habitable, planet. In 10-20 years time, though…….