How well do you know the Sun? It hasn’t always looked the way it does today. Billions of years ago, the Sun was fainter but also more active, throwing out huge flares of radiation in powerful tantrums. This “young Sun” helped shape the evolution of life as we know it. By understanding what our Sun was like when life emerged on Earth, scientists can look to other stars in the galaxy and think about whether life could emerge on planets there, too. Vladimir Airapetian, scientist at NASA’s Goddard Space Flight Center, explains what researchers hope to find as they gaze beyond our solar system.

Jim Green: We know the light from the Sun is so important to us today.

Jim Green: What is really the evolution of our Sun and how it has affected life here on Earth?

Vladimir Airapetian:The lack of oxygen was one of the most important conditions to start biological molecules.

Jim Green: Hi, I’m Jim Green, Chief Scientist at NASA, and this is Gravity Assist. On this season of Gravity Assist we’re looking for life beyond Earth.

Jim Green: I’m here with Dr. Vladimir Airapetian and he’s an astrobiologist at the Goddard Space Flight Center. In addition to that, he’s a full professor at American University. Vladimir has been analyzing solar storms and how they affect planets. So today we’re going to talk about the effect of our Sun on life here on this planet, and what it tells us about possible life on other planets. Welcome Vladimir.

Vladimir Airapetian:Thank you. It’s great to be here.

Jim Green: We think of the Sun as a constant, you know, constantly shining in the same way. But is that really true?
Vladimir Airapetian:Well, our Sun is a magnetic star. From time to time, large bundles of magnetic field emerge to the solar surface and form Sunspots, the regions of enhanced magnetic field that causes activity known as active regions. Strong magnetic field in these regions move due to the surface convection and at some point can generate magnetic tornadoes and hurricanes. That can generate flares by transforming magnetic energy into heat and kinetic energy through magnetic reconnection, or by snapping and reconnecting magnetic field lines.
Vladimir Airapetian:So field lines break and rejoin fast and expel billions of tons of materials, unleashed in a ejection called the coronal mass ejection. As this coronal mass ejections travel to Earth and other planets, they disturb their magnetic bubbles called magnetospheres and generate magnetic disturbances known as magnetic storms.

Jim Green: So this is really an exciting field for us. In fact, you can see the excitement that’s going on in the Sun with these solar storms, so to speak, by looking at a variety of our satellite data that’s online. So, uh, we know that the Earth and the Sun are about 4.6 billion years old. But what do we know about the young Sun and what was it like, how active was it?

Vladimir Airapetian:It was an extremely magnetically active star, rotating up to 10 times faster than it is today, producing large Sunspots which the size is 10% of its surface and generating large and frequent flares.

Vladimir Airapetian:We see super flares on young stars in abundance from Kepler mission. Recently we found a couple of super flares from Kappa-1 Ceti, a twin star of our Sun at the time when life started on Earth.
Jim Green: When you talk about super flares, how big are they? What are we really talking about?
Vladimir Airapetian:Well, the super flares can be as energetic as a 100 times more energetic than the largest solar flare ever observed on our Sun today, in current times.

Jim Green: So just how important was the young Sun to life here on Earth?
Vladimir Airapetian:Well, it was essential component in producing life because life needs three essential requirements. The first requirement is to have liquid water and the Sun was also the one important contributor to that because it produced greenhouse gases. The Sun was a faint star, it was magnetically active star. But with 30% fainter lens today.
Vladimir Airapetian:So the so called faint young Sun’s paradox was in place, how to explain the liquid water under the young Sun, when it’s supposed to be an icy ball. So therefore, we think that the Sun produced abundant nitrous oxide, one of the gases that help to heat it to the temperatures to allow liquid water. The second requirement is to have a chemistry and an atmosphere that can eventually be broken into those complex molecules.
Vladimir Airapetian:Those requirements are important in order to accumulate those molecules and make them mature to the complexity. Become more and more complex on the surface. So it’s a complex process.

Jim Green: We know that early life started on Earth about 3.8 billion years ago, but the atmosphere at that time had little or no oxygen. What else is happening to that early Earth and the life that may have started here on Earth?
Vladimir Airapetian:That’s an amazing question. The point is that the lack of oxygen was one of the most important conditions to start biological molecules. Because oxygen oxidizes the simple molecules and doesn’t help to create complexity. Complex molecules need a little oxygen like carbon monoxide for instance, instead of carbon dioxide. So we say that the atmosphere was mildly reducing, meaning that it had some hydrogen, it had carbon dioxide, a little bit methane, nitrogen, that was the one essential component of life.
Vladimir Airapetian:That helped to create the major gases like hydrogen cyanide, the feedstock molecule of life, formaldehyde and other molecules outer with that should be present abundantly in the gas phase in the atmospheres. So the future observations, we need to look for those signatures. And then later on when the life started, when the chemistry became biology, that created methanogens. The simple organisms, as you correctly stated, that basically didn’t need any oxygen. They absorb carbon dioxide and release methane. That’s why they’re called methanogens.

Jim Green: Well, it sounds like this breaking apart and recombination can generate some really poisonous gases. How does life come out of that?
Vladimir Airapetian:Hydrogen cyanide is a really poisonous gas. It’s the matter of national security. Today, you cannot buy it in stores, but it turns out that this hydrogen cyanide, if you add up to the simple molecules, you create more and more complexity. The poison early in life becomes treasure of life today.

Jim Green: So how do we know so far back, that the Sun was really active? How do we tease that out?
Vladimir Airapetian:Oh, that’s a fantastic question, and the point is that large flares produce coronal mass ejections that ignite solar energetic particles and those energetic particles penetrate into the atmosphere. They break molecules and they create the carbon 14 isotopes. So out of carbon uh oxygen and nitrogen, and this carbon 14 joins the oxygen, creates the carbon 14 uh carbon dioxide and absorbed by the trees. So we see traces in the tree rings.
Jim Green: Wow. That’s interesting. We always knew that the tree rings where you see a ring every year that it lives and it grows. The thickness of that ring tells us a lot about that year’s input, which is the Sunlight and these heavy particles that come streaming through our atmosphere.

Jim Green: So during a star’s life, they are very active when they’re young, what happens next?
Vladimir Airapetian:Oh, then they lose their steam because the Sun rotates slower, it produces much weaker magnetic field. So producers smaller flares, less frequently. Some becomes a mature star. Any mature system behaves a little bit quietly. So that’s what we have today.
Jim Green: But even today, a quiet star, we know that our Sun has really put out some fantastic coronal mass ejections.

Vladimir Airapetian:Recent observations of mature solar analogs, like our Sun today showed the generation of very strong flares a 100 times stronger that we observe today. That suggest that in the future we can observe a catastrophic event and we need to understand its impact on the whole system, on a system starting from magnetosphere to our civilization, that can produce the large atmospheric currents, all the way producing the changes in a stratospheric ozone that will increase the radiation, the extreme UVB and UVC emission coming to the surface and actually affecting crops, affecting a lot of life forms on our planet. Because the effect can last up to a year or even longer.

Jim Green: Do you think we can find a young Earth in our local neighborhood of stars?
Vladimir Airapetian:Well that’s amazing question and we’re looking for, so I hope that, well we need to look through K2, that extended mission of Kepler that looked at the stellar young solar clusters. Unfortunately Kepler couldn’t observe it because it was pretty small telescope and also, stellar vulnerability of those young stars mimics the planetary signatures too. So we need to work a little bit harder in order to uncover the signatures of exoplanets around young stars.
Jim Green: So this is really a fascinating topic. We really need to do looking at the Sun and how it is evolved and how our planets evolved and therefore match that with how life here evolved on Earth. Then go find places near our Sun, near our neighborhood of the galaxy where we expect a lot of planets to be created and find that object that is not just Earth size but Earth-like. So we have some exciting observations coming up.

Jim Green: We have a whole variety of stars in our galaxy. Are some better for creating solar systems and looking for life than others?
Vladimir Airapetian:The point is that first the planet needs to be in a habitable zone and the cool stars, smaller stars, they have much narrow habitable zones. The planet needs to be much, much closer. That means that they should be exposed to the huge fluxes of X-ray and extreme UV emission and the flare emission, that is bad for, too much of a good thing it’s a bad thing.

Jim Green: We talk about habitable zone, but what does that really mean?
Vladimir Airapetian:Well, the habitable zone classically originally was introduced as a shell around a star, where so called Goldilocks zone, where the temperature is not too cold and not too hot. Allows the water to exist in liquid state. But then later we found that that’s only one condition and then you need to have the zone not too close to the star, to make sure that the planet has a thick atmosphere. So therefore that’s another factor to space weather important factor in addition to the classical habitable zone.

Jim Green: So small stars have problems of having the planets too close. Well, what about the really large stars?
Vladimir Airapetian:Oh, well, we’re talking about the stars a little bit hotter than the so called M dwarfs and cold cold stars like K type star. So the stars are slightly cooler than our Sun probably a sweet spots for life. Because the planets in the habitable zones a little bit closer at the distance of might be a Mercury or between Mercury and Venus, but still, I mean, they exposed to a lot of radiation, which is a good thing, but still they can preserve their thick atmospheres, which is a big, big requirement.
Jim Green: What about the A and B stars, the really big and really hot stars?
Vladimir Airapetian:A and B stars, they’re one of the worst cases for life because they produce so much emission. So the habitable zone should be located farther away where you don’t see any materials. You need to have some material not to build a planet first and have essential chemistry for this planet to have life. So I would imagine that, they should have very, very little material to form planets in the first place.
Jim Green: So when we’re out there looking for life at different stars, we have to really be choosy about what stars could actually support a solar system where life may exist?
Vladimir Airapetian:Absolutely. A star is the first clue for a life on an exoplanet. First, the existence of an exoplanet and then life and habitability.
Jim Green: Vladimir, I always like to ask my guests to tell me what was the event or the place or person or thing that really got them so excited that forced them to become the scientist they are today? I call that event a gravity assist. So Vladimir, what was your gravity assist?

Vladimir Airapetian:My gravity assist had in my childhood, three massive brains. I would say. The first one is the head of the Amateur Astronomy Club when I was 10 year old. I was infected with astronomy and Mars was one of the amazing planets that I was dreaming about to understand whether life is possible on Mars or was possible on Mars. So, as soon as I graduated from the university, the second person who made my life to turn around was [astrophysicist Viktor] Ambartsumian, who was the head of the Byurakan Observatory in Armenia. So I turned my attention to the young, to the young stars and eventually I realized at some point that the Sun is a star.
Vladimir Airapetian:So if I know the life of young stars, I can uncover the life of young Sun. The third person who made a big difference was Stirling Colgate that was Los Alamos National Laboratory who passed away a few years ago. So, those three people created this environment that made it impossible not to think about astrobiology.
Jim Green: That’s great. Well, join me next time as we continue our journey to look for life beyond Earth. I’m Jim Green and this is your Gravity Assist.

Credits:

Lead producer: Elizabeth Landau

Audio engineer: Manny Cooper