Postdoc Spotlight: Exploring Exoplanets with Fabo Feng

Thursday, January 30, 2020 

Fabo Feng is a self-described exoplanet explorer, but he can't jump on a spaceship to discover distant planets in person. Instead, he builds complex algorithms and uses the radial velocity method to try to find Earth twins.

Before becoming a postdoctoral fellow at Carnegie Science, Feng received his Ph.D. from the Max Planck Institute for Astronomy in Heidelberg, Germany, where he first became interested in exoplanets while considering astronomical effects on Earth's habitability. From 2015 to 2018, Feng conducted postdoctoral research on exoplanets at the University of Hertfordshire in England, where he first developed Agatha, an algorithmic framework used to efficiently disentangle exoplanet signals from the noise caused by stellar activity.

Since 2018, he has been working at Carnegie Earth and Planets with staff scientist and exoplanet pioneer Paul Butler to continue his search for potentially habitable exoplanets.

In this postdoc spotlight, Feng answers questions about his recent paper that reports on 16 exoplanets: five new exoplanets, eight planet candidates, and the confirmation of three previous candidates. He also explains how he finds exoplanets, discusses why he thinks exoplanet research is important and gives advice to young scientists. Feng's responses are lightly edited for clarity.

Carnegie Earth and Planets Laboratory (CEPL): What exactly do you do here at the Carnegie Science Earth and Planets Laboratory?

Fabo Feng (FF): I use the so-called radial velocity method to detect exoplanets, namely planets orbiting around other stars.

I came to Carnegie Science because it is well known for its cutting-edge research on planetary science. In particular, I share a common interest with Drs. Paul Butler, Alan Boss, John Chambers, and other faculty members.

CEPL: What first sparked your interest in science?

FF: I remember as a child, the first time I looked at stars in the night sky, I was amazed by how enormous and how deep the universe is. It seemed to me that someone was looking at me. 

This kind of wonder, curiosity, and mysterious impression of the universe sparked my interest in science. Since then, I began to read lots of books about science and became more and more interested in science. 

Then, when I was a high school student, I read Steven Hawking's book titled "A Brief History of Time." This book is so profound and intriguing that I decide to be a scientist right then!

CEPL: How did you get interested in studying exoplanets?

FF: During my Ph.D., I studied the influence of astronomical phenomena on planet Earth.

While I was studying the astronomical influences on the Earth's habitability, I began to wonder whether there are other habitable worlds like Earth, whether there is any extraterrestrial life, and, even further, if there are intelligent beings out there. Motivated by this curiosity, I began to develop various algorithms to detect potentially habitable planets during my postdoc in the UK. Then I continued to hunt these exoplanets here in the US.

CEPL: Why do you think finding potentially habitable exoplanets is important?

FF: As you know, Earth is the only known place where life exists. To understand human's place in the universe, we want to know whether there are other planets as habitable as the Earth, whether there is life on them, and whether there are intelligent beings. To answer these questions, we first need to find exoplanets that are similar to Earth and which are also potentially habitable. 

CEPL: What do you mean when you say a planet is in the "habitable zone"?

FF: The minimum condition for life is probably the existence of liquid water on the surface of a planet. Scientists use this criterion to define the so-called habitable zone, where the planet is not too hot nor too cold for liquid water to exist. 

CEPL: Most exoplanets have been detected in habitable zones orbiting M stars, also known as red dwarf stars. How do these M stars differ from our Sun, and does this affect habitability?

FF: M stars are smaller and much fainter than the Sun. Thus, for planets to be in the habitable zone, they have to be on a closer orbit around their M stars, and that means most of them are likely to be tidally locked. Being tidally locked is a problem for habitability.

To understand what it means to be tidally locked, think about the Earth-moon system. Only one side of the moon faces the Earth. On a tidally locked planet, one side is too hot, and the other is too cold. So, despite the possibility of liquid water, they are unlikely to be habitable.

In our most recent study, we have found planet GJ 180d, which is in the habitable zone and distant from its star, so it is unlikely to be tidally locked and thus is more likely habitable!

CEPL: In your recent paper, Paul Butler and you discovered two "super-Earths" in addition to a "cold Neptune." What's so exciting about these discoveries?

FF: Actually, we reported 16 exoplanets. The two super-Earths and one cold Neptune are only the most interesting ones among them.

To me, the most exciting discovery is that of the super-Earth GJ 180d, which, as I said before, is unlikely to be tidally locked and thus is more likely to be genuinely habitable!

In the same study, we discovered another habitable zone world called GJ 229Ac. GJ 229Ac is the nearest habitable-zone planet located in a binary system, which includes an M star and a brown dwarf (so-called failed star).

We also discovered the nearest, coldest, and widest (i.e. largest orbital period) Neptune-like planet, GJ433 c.

The cold Neptune is unique because it is located in an unexplored region, discovering these planets is the first step to finding Solar System analogs. 

As you can see in the image I included, GJ 433 c is located in an unexplored region where the “cold Neptunes” are. The discovery of these objects is important because we want to compare them with our Solar System’s Neptune. The discovery of such a planet is the first step to find Solar System analogs to answer the question of whether our Solar System is unique.

CEPL: So, how exactly does one find a planet? It's not like you can go to space to visit them.

FF: There are many methods we can use to find planets. For example, I use the radial velocity method, which is based on the fact that a planet's gravity affects its host star’s orbit. The stars themselves have orbits that mimic the movements of the planets orbiting them. Thus, when the star is moving towards us or away from us, it is periodically blue or red-shifted. We can detect and measure this periodic Doppler shift, and from there, we can detect orbiting planets.

There is also the transit method. When a planet moves in front of a star, it will block part of the light emitted by the star and cause a dip in the measurable light. From the periodic dips, we also can detect planets.

For our most recent paper, we used the radial velocity method to find the 16 exoplanets.

CEPL: You found more than 20 exoplanets in 2019. That's a lot of exoplanets. What makes your approach different or unique?

FF: I have spent four years developing advanced algorithms to find planets reliably and efficiently. When we get the radial velocity data, there are multiple signals in it, including planetary signals, stellar activity, and instrumental and atmospheric effects.

My algorithms can detect planetary signals reliably. In particular, stellar activity is sometimes much stronger than planetary signals. To resolve this, I have used the so-called Goldilocks principle to correct for stellar activity. I have also developed comprehensive tools to disentangle planetary and stellar activity signals.

All these algorithms are highly automated, which means I can find planets efficiently. This is the reason why I was able to discover more than 20 new exoplanets and planet candidates last year alone.

We believe that all relevant data should be made public once the paper is published. It is also the rule in this field so that others can analyze the data independently to test our conclusions.

CEPL: Where can people go to find this open-source data and software?

FF: You can find the data and the methods we used to analyze it on Dr. Paul Butler's website. You can also freely download Agatha and PEXO, two open-source programs that I developed to find exoplanets efficiently. Both can be found on GitHub.

CEPL: What is the ultimate goal of finding these exoplanets?

FF: For different astronomers, there might be different goals for finding exoplanets. Some people use the exoplanet data to understand how planets formed and how they evolved. For others, they just want to study exoplanets in detail. For example, they want to examine their atmosphere and composition.

For me, the ultimate purpose is to understand our position in the universe by answering the questions of whether the Earth is the only habitable world and whether the Solar system is unique, whether the life on Earth is unique, and whether there are other intelligent beings like humans.

To answer these questions, we need to find more exoplanets, especially Earth-like planets. Eventually, we may even be able to image them directly to look for signs of habitability or detect signals that are generated by life. It is just like living in a community and gradually making more and more friends and knowing more and more neighbors. These planets are neighbors of the Earth.

CEPL: Do you have a favorite planet?

FF: My favorite planet probably is the Earth! My favorite exoplanet is probably Proxima b, which is orbiting Proxima Centauri, the closest star to the Sun.

An illustration of 'Oumuamua, the first object we've ever seen pass through our own solar system that has interstellar origins.  Credit: NASA Goddard Space Flight Center

CEPL: What is one of your favorite memories as an astronomer?

FF: For me, I think it was getting to study 'Oumuamua, the first-ever interstellar object to be found in our Solar System! 

`Oumuamua is very different than any other object in our solar system; it is highly elongated, rotating fast, and very rigid. It's so weird that somebody even proposed that it could be an extraterrestrial spaceship!

To find its natural origin, I simulated the motions of millions of stars. I found that many of the stars belonging to the Pleiades moving group pass by 'Oumuamua slowly at a short distance. So, I concluded that 'Oumuamua is a young asteroid that had been recently ejected from the Pleiades moving group.

It gets better! I wrote an article about 'Oumuamua in the online journal "The Conversation," and more than 200,000 people read it! I was contacted by many journalists and laypeople to discuss the origin of 'Oumuamua. It is such a great experience to be involved in the study of something that has never been studied before. I am happy to be one of the pioneers to study interstellar objects.

CEPL: If you could meet one of your science icons, dead or alive, who would it be?

FF: I guess it would be Johannes Kepler, a German astronomer who discovered the laws of planetary motion. Before him, there was only chaos in how we understood the heavenly bodies. After him, people firmly believed that the universe could be described by mathematics. His laws are still used by modern astronomers to find exoplanets. The famous Kepler satellite is also named after him. Kepler said, "I am merely thinking God's thoughts after Him." So he claimed to be the discoverer of the laws given by God.

CEPL: What research do you hope to pursue in the future?

FF: I would like to find Earth twins and exomoons and apply my recently developed software PEXO to detect gravitational waves and test general relativity.

CEPL: What do you think will be different in your field of study, say, 100 years from now?

FF: Science advancement is hard to predict. For example, no people in the 19th century would have been able to predict quantum physics and general relativity theory.

Although it is difficult to predict these genius ideas, there are some things we definitely could achieve within 100 years. From an optimistic point of view, more and more exoplanets will be found, I believe we will find almost all planets down to the size of Mercury in the Milky Way by the end of this century. We will also find lots of exomoons around them.

We will probably know whether they are habitable by studying their atmosphere, geology, and weather. We will have a complete catalog of habitable planets and know whether our Solar System is unique in its structure. If there is life out there, we may also detect it through spectroscopic observations. The search for extraterrestrial intelligence (so-called SETI) might answer the question of whether human beings are unique in the local universe. We might even be able to send robots to the nearest neighbor, Alpha Centauri, to study their planets through light sails enabled by laser beams.

CEPL: Do you have any final words of advice for young scientists?

FF: Science is not easy, but it is highly rewarding. As a young scientist, you will find joy, meaning, and wonder in your discoveries. Such discoveries and achievements should shape our character and life so that we become more humble, more mature, more thoughtful, and more open-minded.