They are planetary bodies made up of solid rock and surely water … a lot of water. Their size ranges from one to ten times that of Earth, and they orbit in solar systems located dozens of light years away from ours. The first proper was discovered in 2005, but by then Diana Valencia, a young planetary scientist at Harvard University, had already been developing possible models of its internal structure and geophysical evolution for some time. He christened them super-Earths. And boy did the term catch on! In just two years, these new worlds have become the object of research for a select handful of researchers and the raison d’être for two unmanned space missions. Angela Posada-Swafford has spoken with her in Cambridge, Massachusetts, about these new extrasolar gems and the great interest they are awakening among astronomers around the world.
-You were making models of the structure of these planets before they were even discovered! How can it be possible?
-It’s true; I started making my models four years before the first one was discovered. At that time, many gaseous planets had already been found, at least 200, so I suspected it was only a matter of time before the rocky ones appeared. This field of research is growing at an impressive speed. So much so that a student who is in his doctorate years, as was my case until recently, can contribute a lot, and that is something that hardly happens in other scientific areas.
-The super-Earths seem to be on everyone’s lips today, from astronomers to the media. How did you come up with that term?
-When we started this project we only thought of worlds like ours, but of greater mass. Thus was born the term super-Earth. As we worked with other experts, we realized that the planets we were looking for could also have a huge ocean, so for a while we thought about modifying it (calling them, for example, oceanic planets), but finally, Cape Earth also has water, we decided to leave the name as it was.
-What does it take for a planet to be classified as super Earth?
-Well, that it has between one and 10 times the mass of our world and that it is made up of rocks and / or water. Gaseous bodies would not be included in this classification, although some media often insist on calling anyone with masses higher than ours. For example, the first one to open the list, which was named Gliese 876d (GJ876d), is 7.5 times larger than Earth. It is very exciting to have found a “cousin” of our planet, with very similar physical characteristics, in another solar system. So we use it as a basis for developing our theoretical models.
-How did you get involved in the study of these celestial objects?
-I came to the conclusion that there are characteristics that seem unique to Earth because our planet is more massive than Mars, Venus or Mercury. I decided to investigate to what extent mass influences the properties of these bodies, or if they are due to their evolution. In other words, if Mars had the mass of Earth, would it be more like it? And if so, would there be life in it? Then we did not know rocky planets similar to ours but with more mass and our experience was limited to the Solar System.
-How does the distance at which they are from their star affect these rocky planets?– That is an interesting question where there have been many surprises. For example, when a good number of gaseous planets were discovered whose period of rotation around their sun was only three days, we all had to rethink what we knew about how these bodies form. The same thing happened to us with super-lands. For example, GJ876d has a period of two days. Although we have yet to verify our hypotheses, today we suspect that the planet actually formed farther from its star, where it may have acquired a lot of water, and then migrated into its system. Of course, the distance at which it can be found from its star has a limit. After this, the environment is too hot for this element to exist, but before it, conditions are cold enough for it to be palpably condensed by ice. Thus, we can find super-Earths near and far from its parent star.
-How do you know the composition of one of these worlds?
-The two key elements are mass and radius, and we were the first to propose a relationship between these two variables. If we take both into account, we obtain measurements that give us clues about the composition of the planet, that is, we can know more or less what it is made of. For example, if the planet in question has a radius of 8,000 kilometers, its composition cannot be the same as that of the Earth, which has 6,400. I think our most important discovery has been to detect that these worlds have a good chance of being oceanic if they exceed 12,000 kilometers in radius, since the water is very light. And it is that all this also has to do with the pressure range. The pressures inside a planet whose mass is 10 times that of Earth are very high. Thus, even if it had the same diameter, it would be possible to put more material. We have also integrated into the models another class of planets, the superMercuries. In them, the nucleus is immense, precisely as it happens in the smallest of the planets of our system, where it occupies 60% of all of it. In addition, our super-earth models allow us to work with any composition, from the silicon, which forms most of the mantle, to the iron in the core, and other elements such as nickel, titanium, liquid water or different states of ice.
-But radio can also tell us where the planet was formed …
-Exactly. If it is close to its sun, a large radius is considered proof that the planet migrated into the system and that all the water evaporated. It is a measurement that tells us a lot about all the dynamism that existed during the formation of the planet.
-What do your models reveal about the possibility of plate tectonics in these super-Earths?
– {mosimge} Earth is the only planet in the Solar System that has plate tectonics. Studying them we have come to the conclusion that as the planet has more mass, the pressures that want to deform the lithospheric layer are greater. At the same time, and contrary to what was believed until now, the larger the planet, the thinner the plate, therefore it is easier for it to break. That is, a planet larger than Earth is more likely to have plate tectonics than a smaller one.
-And what do they tell us about the formation of these objects?
-The larger the planet, the more lightweight material it is capable of attracting. If it becomes very massive, say about 15 times the Earth, it can start to retain helium and hydrogen, and it will soon become a gas giant, like Jupiter or Saturn. But if it does not have enough mass, it will not have the necessary gravitational attraction to retain the gases and it will remain like Earth, with a very incipient atmosphere, or like Mars, which hardly has. In fact, neither the terrestrial nor the Venusian are primordial. They were formed as a consequence of secondary processes. In the end, to refer to super-Earths, we ended up deciding that their mass would have to be between one and 10 times greater than that of our planet, since if it were between 10 and 20 we would find ourselves faced with incompatible conditions for the formation of rocky planets.
-What is the best way to detect a super Earth?
-Actually there are several and they are all complex. This is why so few rocky extrasolar planets have been discovered so far. One is the gravitational microlensing method, an effect we might call “Einsteinian” in which the gravity of a foreground star magnifies light from a more distant one. If the former owns a planet, its gravity can distort the star’s light, revealing its presence. The “transit” technique, for its part, serves to obtain the radius of the planet and occurs when it forms an eclipse in front of a star, so that the luminosity of the star decreases. Mass, meanwhile, can be measured with the doppler shift technique. If the planet is very massive, the disturbance around the center of mass is such that the spectrum of the star can be observed moving closer and further away. The problem is that if the planet is small these effects can hardly be appreciated. But it is only a matter of time. Detection technologies will make it possible to see the phenomenon more easily and detect many super-Earths. There are, for example, the telescopes of the Bells, in Chile. They are so precise that they can “see” the stars moving as slowly as a person walks. And they are thousands of light years away!
-What about the Corot and Kepler space missions, specially designed to detect rocky extrasolar planets? Have you been involved in them?
-Corot is a French telescope, developed in collaboration with the European Space Agency, which was launched in 2006. It is prepared to find planets that have between 8 and 10 times more mass than ours. We are following the mission with great interest and we hope that the first measurements will begin to arrive. NASA’s Kepler mission will depart in 2008 and will be in orbit for three years. For me it is very exciting, since my planetary models are linked to it and we will be able to work by processing the data sent directly by the telescope.
-Discovering new Giant Lands seems like a challenge for someone whose career is just beginning.
-It’s absolutely fascinating. Actually, the big challenge is finding something that we can measure. We could put any of our models on the table and start theorizing about the characteristics of this or that planet. But that wouldn’t mean anything if we couldn’t apply it somehow. These missions will allow us to do so.
-Now all we need is the science fiction novel …
-A Canadian author has suggested that I put my name to one of the planets in the book he is writing! Sounds like a really fun idea to me.
Angela Posada-Swafford
