Paul Butler seems like a typical carefree, strong and good-natured American, but he has something special: he is part of that tribe of astronomers who every few days announce the discovery of new extrasolar planets, those that orbit around a star that is not the Sun. His dedication has a point of romance, not in vain he spends a hundred nights a year observing the stars. But he remembers that his work takes place rather “like that of a Formula 1 mechanic”, dedicated to improving the planetary location technologies without which the map of the universe could not have been dotted with so many new colleagues from Earth.
-What is your strategy to hunt planets?
-The Milky Way extends to a distance of 100,000 light years and we focus on distances of 150 light years. Thus, if the Milky Way were Spain, we would only be analyzing Barcelona, Madrid or Seville. Our objective is the planets that orbit around the 2,000 stars closest to the Solar System, out of the total of 200,000 million in our galaxy. We work with them because they are the brightest stars and we can analyze them better. After we detect them, they can be identified with other technologies, such as astrometry and direct imaging. The planet hunters are the outpost, the explorers of the new territory, like Columbus and the other European navigators who conquered America.
-But Columbus was looking for something he couldn’t find.
-Well, the same thing happens to us. When extrasolar planets had not yet been discovered, we believed that we would find objects similar to those in the Solar System, rocky like Earth or gaseous like Jupiter. The reality has been that we have found stars with attributes that no one would have imagined. For example, 51 Pegasi b, the first extrasolar planet found in 1995, is the size of Jupiter but has a tiny orbit, with a period of only four days around its star. That is, it is closer to it than Mercury is to the Sun. That is why it has terribly high temperatures.
-How did you contribute to the discovery of that first extrasolar planet?
-On October 6, 1995, a Swiss group from the University of Geneva, our main rivals (laughs), made the discovery. At that time many scientists did not believe in its existence. Skepticism prevailed because during the previous fifteen years many astronomers had mistakenly announced the discovery of an extrasolar planet. The strange typology of the star also influenced, with its Jupiterine dimensions and its small orbit. My colleague Geoffrey Marcy and I from the extrasolar planet search group at the Universities of California and Carnegie were fortunate to have the telescope the week after the announcement and were able to verify this. It took us four nights, as many as there are days in its orbit. Our confirmation caused quite a stir, and after that we found the next five extrasolar planets.
-And now, how many planets are known outside of our Solar System?
“So many have been discovered that I don’t keep track!” The figure is around 250, and between the Swiss group and ourselves we have found around 85% or 90% of them. Every week or every fortnight it is announced that another has been located. And both groups have more on the waiting list, we are prepared to publish new discoveries. This was unimaginable when, not so long ago, we still hadn’t found any.
-How could our knowledge be accelerated so much?
-We are the beneficiaries of the great advances in computing and digitization, products of the work of very intelligent people. In 1993, it took me weeks or months to analyze a one-night astronomical observation on the computer. Now I do it in five minutes thanks to Moore’s law – the computing principle that chips double their capacity every two years. Anyway, so far we have only been able to find out that 10% of the stars have planets. When our technology improves, we will surely see that there is more.
-Actually, you still cannot see the extrasolar planets directly.
-No, we work with indirect techniques and what we perceive is the light of a star. Around it a planet orbits that we do not see, but we know that it gravitationally influences its movement. If we decompose the star’s light using high-resolution spectroscopy, we can deduce the presence of a planet, its orbit, and even its mass. For all this process we use very powerful optical telescopes. The images they take are entered into the computer through CCD detectors, like those of any digital camera, to be analyzed with the spectrograph. The problem is that such distant light sometimes translates into little more than a bright pixel on the computer, but its analysis offers us a lot of data.
-So, a good part of your work is not with the telescope, but with the PC.
-Many believe that, since I am an astronomer, I am always looking at the stars, but most of my time is spent thinking of little technical ideas to improve the precision of our observations. It is a job similar to that of the mechanics of a racing car, trying to get more speed for their car. For example, most of the planets that we have discovered are the size of Jupiter – with a mass that is 300 times that of the Earth – but that does not mean that the universe is like that, but that our technology is more prepared to locate them. .
-Can you explain this idea in a little more detail?
-In the beginning we only detected large planets with small orbits, which were easier to observe with the margin of error with which we operated, of 300 meters per second. The smallest planets move at twelve meters per second. In recent times, our group and the Swiss team have developed precision technologies that allow us to detect movements of ten meters per second, and we are approaching speeds of one meter per second. So we can discover smaller planets, similar to ours, like one we found in 2005 that is only 6.5 times more massive than Earth.
-When will it be possible to see these Earth-like planets directly?
-I’m 46 years old and I hope to see your photo (laughs). Ten years ago it was not even known that the closest stars had planets. NASA and ESA dream of sending telescopes that take these snapshots into space, although there are still 15 or 20 years to go. When it is achieved, we will not see much, because the photo will be a single pixel, but it will be enough for the spectral analysis to allow us to decipher the chemistry of the planet. It will be exciting.
-The discovery in 2007 of the extrasolar planet HD 189733 b has raised many expectations due to its similarities with the conditions of terrestrial life. Are there reasons to think that life can exist in it?
-With an orbit of only two days around its star, HD 189733 b is an uninhabitable spit. Anyway, the discovery of water vapor in it is exciting. It is already known that water is a common substance throughout the galaxy, and this discovery suggests that it is also incorporated into extrasolar planets.
-Do we know the universe better now?
-Yes, but the main conclusion we have reached is that everything we knew ten years ago was wrong. Humans are incredibly ignorant, and also arrogant. What we ignore about the universe is much more important than what we know about it.
Jose Angel Martos
