Space missions looking at worlds beyond the Earth
Article by Anastasia Kokori, who has participated in a Europlanet expert exchange programme with the department of astrophysics at UCL.
It has been more than twenty-five years since the first planet was detected beyond our Solar System. Today, we have discovered nearly 4,000 exoplanets, varying from small rocky planets like the Earth to gaseous giants like Jupiter.
Prof. Giovanna Tinetti, a researcher in the field of exoplanets working at UCL (University College London) explained: “Now we know that, statistically, we should expect at least one planet around every star in our Galaxy. We are literally talking about billions of exoplanets. So, with this information, the questions we have now in mind are what do these planets look like, and how do they form and evolve. These are, I think, the kind of questions we will try to address in the next decade.”
Small planets like the Earth are many times more common in our Galaxy
It is beyond doubt that the Kepler mission was a big breakthrough that brought us closer to answering fundamental questions. It discovered more than 4000 candidate exoplanets before the end of its mission in 2013.
Prof. Dimitar Sasselov from Harvard University, a co-investigator of the Kepler mission, said: “Kepler was trying to find the statistics about how common are planets like the Earth. Small planets like the Earth are many times more common in our galaxy than we even dreamed before. So, the planets that we know about that belong to the habitable zone are many more now.”
Being in the habitable zone, however, doesn’t mean that these planets are habitable or that they host life. Prof. Sasselov added: “If we don’t understand the basic geochemistry of these planets we can’t even understand the search of evidence for life.”
There are many questions to be answered and many mysteries to be solved in the following years.
It is important to study the atmospheres of exoplanets
Knowing that there are a huge number of exoplanets out there, we now need specifically designed missions that can devote their time to exoplanet research. As yet, we don’t have much information on exoplanets’ compositions, their atmospheres and other details that could help us answer our main questions. Investigating a large number of planets is now possible with the technology we have developed and we are starting to be able to provide answers and develop a taxonomy of planets — an organized list of planet categories.
One such dedicated mission will be ARIEL (Atmospheric Remote-sensing Exoplanet Large-survey), one of three candidate missions currently being considered by the European Space Agency (ESA) for launch in 2026. The main goal of the mission is to study the atmospheres of hundreds of planets and understand how they form and evolve. The main difference compared to other missions is that ARIEL will not detect new planets, but it will focus on the atmospheric composition of already known exoplanets.
According to Prof Tinetti, the principle investigator of the mission: “ARIEL will use spectroscopy to analyse infrared light from distant solar systems. This is a different focus from other space missions that have been planned and approved by the ESA or NASA to date. Additionally, it will study the atmospheres of several hundreds of exoplanets. These are the characteristics of ARIEL that make it unique.”
ARIEL will focus on the atmosphere of exoplanets, which is the only directly observable part from which scientists can extract additional information beyond the size, the mass of the planet and its distance from the host star.
Prof. Tinetti added that: “The atmosphere is made of gas and the molecules in it make it transparent to some types of light and opaque to some others. In this way, we can understand from a remote view what is going on, because we can basically observe the spectral “fingerprints” of the molecules in the atmosphere.”
Detecting more planets in our neighbourhood so we can study them better with ARIEL!
While we have detected a significant number of planets, this still remains tiny given the large scales of our Galaxy and the Universe. Certainly, it is important to detect more planets. The principal goal of TESS (The Transiting Exoplanet Survey Satellite), a NASA mission planned for launch next year, is to detect small planets within the solar neighbourhood. These planets will be very good targets for detailed characterisation of their atmospheres.
Prof. Sessalov said: “TESS is trying to discover the nearest planets that we can study with the telescopes we have. TESS is simply a machine providing targets for the biggest telescopes and for the ARIEL mission. TESS finds the planets, ARIEL studies them. That’s essentially the role of TESS. They work in synergy, but what they do is completely different.”
Searching for life beyond Earth is not an easy task
The realisation that Earth-like planets are very common in the Galaxy, lead to the development of TESS, a telescope that will do a whole-sky survey to find the nearest planets. However, not all earth-like planets should be considered habitable worlds or as the only potential planets for supporting life.
Prof. Tinetti, clarified that: “What we see out there is a huge diversity, and thinking that habitable worlds need to look like the earth is, to my opinion, quite geocentric and probably wrong.”
ARIEL is a mission that will provide scientists with a large database of hundreds of exoplanets and will give insights into the conditions for habitability as well.
Prof. Tinetti highlighted that: “Before we focus our questions on what habitable planets should look like, we need to try understand what is going on with planets. In order to do that, we really need to have a larger sample, not simply tens. At the end of the day we still don’t know what the normality of planets in our Galaxy is. You can’t say “this is a really weird exoplanet” at the moment. It’s weird perhaps compared to our planet but, if you don’t know what planets look like in general, then it is very hard to make any judgement.”
Additionally, Prof. Sasselov pointed out that we don’t know how difficult it is for life to exist, even if the conditions are right. “Some people say that if the conditions and the climate are there, which will be the case for some Earth-like planets, life starts. On the other hand, it may turn out that the Earth is a precious little planet with life on it and there are only maybe a few thousand more in the Galaxy. We don’t know the answer to that and this is essentially what the “origins of life” research is also trying to answer.”
Water is very common in the Universe – you cannot avoid it!
So far, the research in our galaxy has shown that water is very common in the Universe. Usually, water is considered essential for life and habitability. But is this the case?
Prof. Sasselov said that: “Life on Earth works with water as a solvent. The molecules require water as a solvent. But this may not be the only pathway. Until we know whether this is unique or just one of many options, we can’t say anything.”
Essentially, the reason why we find water in our Galaxy very often is that it exists in very large amounts. Water has been found in hot Jupiters, giant planets orbiting very close to their host star. However these worlds cannot be habitable because their temperature is extremely high.
Prof. Tinetti added: “The life as we know it requires liquid water on the surface. Whether this is a real requirement or there are other forms of life not necessarily connected with liquid water, is still a question.”
Exciting collaborative missions such as TESS and ARIEL will hopefully bring us closer to answering fundamental questions of science and society: How do planets form? Are there other planets like the Earth? Could they host life? Is there any other type of life?
The answers will help us better understand the Earth’s and our position in the Galaxy, and the Universe as a whole.