Steven Vogt just finished his lectureIn search of other worldsat the Barcelona Science Museum and a young fan asks for an autograph, as if it were a movie star. Certainly, this American astronomer can boast of being a famous person thanks to his profession as a planet hunter. His words transport the audience beyond the Solar System, glossing over a task that, as he conveys it, is a necessary mission for our future.–After listening to you, it seems that your work is a preventive task for humanity. Are you saying this because of my belief that we will have to leave Earth?
-Yes, nobody thinks about it, but there will come a time when we will have to go, and it is a long road for which we have to prepare.
–Is this flight from Earth a scientific fact?
-In a few billion years the Sun will be larger and, as part of its aging process, brighter and hotter. This will increase the temperature on Earth, which in turn will change the distribution of gases in the atmosphere: carbon dioxide will begin to prevail. This will increase the greenhouse effect, the oceans will evaporate and we will have no water on the planet. The thesis is based on complex models, but which, according to the scientists who develop it, foresees that in a few hundred million years it will be impossible to live in our world. Long before we thought we will have to leave Earth.
–And we will have to move not precisely to the nearest planets …
-Maybe it will be possible for us to live on Mars for a while if we can provide it with an atmosphere and get water. But Mars will also get warmer because of the Sun, and we will need to go further. We have to look for other solar systems in which we can inhabit. They have to be close enough so that we can travel to them in thousands of years, which is how long it will really take to reach them, and establish a presence. The first step is to determine where to go, that is, which stars can sustain life in the future.
–Is that what you are looking for?
– The truth is that right now we do not. I am in the business of finding planets, not life, I do not pay attention to whether they are better or worse to host it. But, once we have found one, we can easily calculate if it is good for it. Depending on the distance that separates it from its star and its brightness, we can know the temperature of the planet. If it is between 0 and 100 ºC, it is in a potential condition to host life. In the last article we published at the end of 1999, we reported the discovery of six planets, of which all but one were in that range that can support life. We are now very dedicated to the smaller planets and I think there may be some exciting surprises. We may find certain similarities to our Solar System, but it will take us at least a year or two to prove it.
–It is hard to believe that characteristics of a planet, such as temperature, can be accurately determined without observing it directly …
-We certainly don’t see the planet at all.
–So how do you make these measurements?
-Actually we do it because of the trace left by the planets. The physics we use to achieve this is very simple. It is based on following your movement. For example, when the light of a known star disappears for a period of time it is because it is eclipsed by a planet. This data allows us to infer its orbit and even its diameter, so we can deduce its size. Once we have it, based on that data and on the angle that the eclipse makes with the star, we calculate the density and mass of the planet. And thanks to these data and its luminosity, we also get to know others, such as its temperature. Following the thread, we will even know what elements it is made of, taking into account that most of the matter in the universe is made of hydrogen and helium. If the planet is very dense, it will surely be a large rock; if not, it will be a gas ball. There are many more interesting measurements, such as the variation in the light spectrum of the star that passes through an exoplanet, whose modifications can offer us information about the atmosphere of the exoplanet, even determine if there are clouds. Ultimately, we get to know everything about him even though we can’t see him. The precision is such that we are sure that, if we took a spaceship and went in search of any one, we would arrive at our destination safely. It is a matter of physics.
–In a way, what they do is reminiscent of when at night we are guided by the shadows to know the dimensions of a person or an object.
-It’s exactly the same kind of trick. And do you know a funny thing? These shadows have reached Earth, hesitant, for millions of years. Many planets have this effect. But they are very difficult to detect, because you have to know where and when to look. From a certain point of view, it is as if we hunted the shadow. And we get a lot of information from it.
–What is the typical robot portrait of the planets you have found?
-They are quite similar to Jupiter. Some have a mass less than that of this planet – around a fifth part – and others, up to five or six times more; they orbit stars, as we do the Sun, and typically trace elliptical orbits.
–Why is Jupiter always a reference point when looking for planets?
-It is above all a good unit of measurement. Thus, for example, we speak of planets of a size of three Jupiters. If you find a solar system with venuses, earths, and Tuesdays in small orbits, you can predict the existence of a Jupiter. And we distinguish between what we call “killer Jupiters”, which have a large elliptical orbit that enters that of other planets and can eliminate them, or the “true analogs of Jupiter”, with a beautiful circular orbit that allows the existence of other planets more small, like ours. But we are finding that all, for now, are elliptical orbit. Although we hope to find one more similar to ours, the truth is that research shows us that we are very special in the universe.
–Could a Jupiter analog host life?
-If it is like Jupiter it is very unlikely, since this is a very dry planet. The Galileo probe tried to detect water and found nothing. In contrast, Europa, one of its moons, is a planet of great promise. Europa has water under the ice, and what maintains it is not the Sun, but the tidal effect, that is, the gravity of Jupiter flexing its moon. It is a very important source of energy, which could eventually liquefy the ice. As is known, for life to emerge what is needed are sources of energy, and here we have one. And notice: Jupiter has 70 or more moons. Nothing makes us think that exoplanets like this do not have many moons and that one is like Europa.
–Should sending a probe to Europe be a priority?
-We could, but possibly a pre-test will be done on Earth. There is a lake in Antarctica, about one kilometer long, which is under the ice, in conditions very similar to those in Europe. We must drill there and see what that water is like.
–Some astrophysicists say that the more we know, the more we realize that we are not that unique in the universe.
-Some, such as Carl Sagan, invoke the Copernicus principle. They claim that humanity has nothing special in the universe and use this principle to predict that there will be planets and life everywhere. But it is not clear that this is actually the case. There are some very special features in our Solar System: the existence of Jupiter with its large circular orbit, rather than a killer orbit; the Moon, stabilizing the Earth’s axis of rotation; plate tectonics …
–What is the importance of the latter?
-The most outstanding thing is the ability they have to keep water in liquid form, thus making life possible, but they are also vital to keep carbon dioxide in its right proportion. After being released into the superficial layers of the sea, it is absorbed by rocks, transported to the interior of the Earth and emitted through volcanic explosions. It seems, therefore, that a cycle regulates carbon dioxide and this is very important so that life does not disappear. The conclusion is that perhaps it is possible that life can arise in many parts of the universe, but perhaps nowhere has the right mix been given. We do not know.
–How do you feel when you think you are discovering planets that you will never be able to visit?
-Well, it’s very sad. It’s like being Columbus, but not having a ship, or even worse, because we know that the planets are there. This is a trick of nature: it allows us to feel all those planets, but not reach them. It is very hard to know that it will take thousands of years to complete missions to any of them.
