Discovery of Potentially Habitable Exoplanets

How are potentially habitable exoplanets discovered and investigated through the use of technologies that utilise gravity and orbital theory?

Introduction

Have you ever wondered how systems like Trappist-1 were discovered? How objects so far away and so unreachable were determined to have atmospheres and water? Well, in this article, you may find the answers to these questions and more importantly may learn something new about our universe.

The Trappist-1 system is located 39 light years from earth, it's star is located in the Aquarius constellation, the system consists of an Ultra-cool dwarf star only slightly larger than Jupiter with seven exoplanets orbiting it. These exoplanets have been of particular interest to scientists as three lie in the stars habitable zone.

"This is an artist's impression of the TRAPPIST-1 system, showcasing all seven planets in various phases. When a planet transits across the disk of the red dwarf host star, as two of the planets here are shown to do, it creates a dip in the star's light that can be detected from Earth." ​(​NASA 2017)

Understanding of investigations

Investigation #1

The scientific report "Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1" (Wit et al. 2018) was published on the 5/2/18. It discusses the possibility for the exoplanets Trappist-1d, Trappist-1e, Trappist-1f and Trappist-1g to have a hydrogen-dominated or a depleted atmosphere. If is the case that one is found to have such an atmosphere, it would enable researchers to eliminate this exoplanet in the search for habitable Earth-like planets. Researchers investigated by monitoring the exoplanets with spectroscopes that would detect near-infrared signatures during transits. If cloud-free, these signatures would confirm whether the exoplanet had a predominantly hydrogen-rich atmosphere. (Wit et al. 2018)

"Transmission spectra of TRAPPIST-1d, e,f and g compared to synthetic H2, H2O, CO2 and N2 dominated atmospheres". (Wit et al. 2018)

This image is a correlation of the data the scientists gathered and it can be used to figure out the atmosphere of each of the planets. As you can see TRAPPIST-1d and e both follow a similar pattern over the blue and green lines (H2O, N2 and CO2). This can then be used to make the assumption that these exoplanets have similar atmospheres to earth allowing them to potentially host biological life.

This report is a collaboration of many well-respected scientists who have a large amount of experience in the area. This along with the highly sort after equipment used (Hubble Space Telescope and South-Atlantic Anomaly) means that they can achieve a high degree of certainty in their results. (Wit et al. 2018)

Investigation #2

The team encountered many anomalies throughout their investigation such as increased scatter and cosmic ray hits during the SAA passes, and programming errors when controlling the HST. These created errors and uncertainty in the results but were correlated with other instruments to rule out most if not all errors that were caused during these anomalies. (Wit et al. 2018)

The investigation "The nature of the TRAPPIST-1 exoplanets" (Grimm et al. 2018) was published on 6/2/18, with the aim to improve our knowledge of the TRAPPIST-1 planetary masses and densities using transit-timing variations (TTV). This resulted in an f-8 times improvement on the previous planetary density uncertainties, with a precision ranging from 5% to 12%. This investigation also found that exoplanets c and e are likely to have mostly rocky interiors while exoplanets b,d,f,g and h either have a thick atmosphere, oceans, or ice and in most cases, large amounts of water. (Grimm et al. 2018)

Mass-radius diagram for the TRAPPIST-1 planets, Venus and Earth. The black dots are the median values. The coloured areas around the black dots indicate error bars for the measurements. The key is representative of the compositions of the planets along those lines. (Grimm et al. 2018)

Assuming the diagram above is correct, TRAPPIST-1e has the closest composition ratio to earth making it the most earth-like exoplanet in the TRAPPIST-1 system. This statement is then further supported when the diagram also shows TRAPPIST-1e as having a very similar mass-radius ratio as earth and venus. All these values hint at TRAPPIST-1e being a rocky world with an iron core as well as comparable gravity to earth.

Strengths

Limitations

This report focuses on the TTV inversion problem which has been studied in great detail and has lots of support from other scientists on being a good method to use to measure aspects of a solar system. This study was also compared against previous studies that had taken place using TTV. (Grimm et al. 2018)

The TTV inversion problem is known to have many issues when it comes to resonant chain systems such as TRAPPIST-1, so throughout the investigation, the scientists made estimates and hypotheses that could be used to compare with data found and correct any obvious errors. TTV also has issues with multi-planetary systems in general with issues such as convergence issues, degeneracies and size of the parameter space. (Grimm et al. 2018)

Investigation #3

The investigation "Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared" (Burdanov et al. 2019) was published on 15/5/19, aiming to clarify and develop dynamical modelling of this system based on timing variations of its exoplanets' transits. The team did this by gathering an extensive photometric data set of 25 observed transits in near-IR J band (1.2µm) with UKIRT and the AAT. As well as the NB2090 band (2.1µm) with the VLT over the course of 3 years between 2015 and 2018. These datasets told scientists the masses and radii of the exoplanets in the system, enabling them to deduce that the exoplanets are mostly made up of rocky compositions.

Diagram of measured transit depths for each transit. Diagram of period-folded transits of TRAPPIST-11 b-g The values are not linear in time but are in order, they exoplanets. Each coloured dot is an individual also have error bars on the measured plane as in this measurement with a black line of best fit and the holo diagram the horizontal plane has no relevance. dots representing median values. (Burdanov et al. 2019)

These diagrams and further datasets cleared up some issues in previous scientific investigations as they provided either confirmation or helped explain why values were wrong. This investigation also updated the transmission spectra of TRAPPIST-1 b-g and discussed stellar contamination enabling future reports to be more accurate in their methods and calculations.

Strengths

Throughout this report, the scientists correlate their gathered results with other well critiqued reports of similar studies. This enables them to see if there are any anomalies in either report and study any that are found. The scientists in this report also used many well-known observatories such as the ESO, AAT and UKIRT. The team also predicted inherent errors. such as the use of ground-based observatories in different areas of the world producing slightly different results due to environments. They mitigated this by using different photometric aperture sizes to obtain more uniform results. (Burdanov et al. 2019)

Limitations

This team was unable to use the Hubble telescope (HST) which would have potentially provided more accurate results as it would not have been affected by its environment as much as other observatories. The team did receive some abnormal results that they attempted to account for by stating that some "transits that were affected by thin cirrus, data gaps or, in one instance, by a possible spot-crossing event". (Burdanov et al. 2019)

Relationship to principles and theory from the NESA Physics course

All of the investigations discussed are linked as they all, in some way, attempt to unveil something about TRAPPIST-1 that would suggest another terrestrial planet that can harbour life is out there.

This is done through Report 1 (Wit et al. 2018) by investigating the atmospheres of exoplanets in the habitable zone of TRAPPIST-1. The report attempts to eliminate exoplanets with hydrogen dominated atmosphere as well as exoplanets with depleted atmospheres, as these types of exoplanets would not be able to support life as we know it.

Reports 2 (Grimm et al. 2018) and 3 (Burdanov et al. 2019) investigated the planetary masses and densities to attempt to determine whether an exoplanet would have a similar gravitational pull to earth. This report also helped analyse the composition of the exoplanets, in most cases, discovering that most had large amounts of water - up to 5% of the exoplanet's mass which is 250 times the amount of water on earth. Report 2 (Grimm et al. 2018) mainly focused on the exoplanets in the habitable zone of the Trappist-1 star whereas Report 3 (Burdanov et al. 2019) gathered datasets on all exoplanets except h, the outermost one, this is because the exoplanet orbits the star much slower than all the rest of the exoplanets in the system.

Diagram of a Transit-timing variation (TTV). (Haghighipour, N 2015)

Through the use of a spectroscope, elements of distant objects can be determined. This has been taught to students numerous times throughout the NESA science courses and has been explained as the separating of white light into a dispersed set of wavelengths. These wavelengths can then be used to determine elements of exoplanets and stars. This was used extensively

Diagram of basic spectroscope mechanics. throughout Report 1 (Wit et al. 2018) as this (Amazing-space.stsci.edu 2020) report aimed to analyse the atmospheres of the TRAPPIST-1 exoplanets. This technique was discarded in the other reports (Grimm et al. 2018), (Burdanov et al. 2019) as it was determined that due to interference from the host star results would be very difficult to analyse.

In Report 2 (Grimm et al. 2018) there is a small reference to Kepler's third law ( T^r3₂ = ^G_4π^M₂ ), which is part of the NESA physics course, and is used to calculate the mean anomaly at the first transit recorded. This report also refers to many concepts taught in the NESA physics course. This includes matters such as eccentricity which is used to determine the stability of the TRAPPIST-1 system considering the exoplanets are in an orbital resonance around the host star. This means that the exoplanets orbiting the star exert a regular, periodic gravitational influence on each other creating a stable system environment. This pattern can then be further confirmed by the Reports (Wit et al. 2018), (Grimm et al. 2018), (Burdanov et al. 2019) when the masses and radii are used to calculate various properties of the planetary bodies in the system. However, all of these are then used in more complex formulas which are not incorporated in the NESA physics course.

Literature review and evaluation of sources

Source #1	Redd, N, (2018), TRAPPIST-1 Worlds Are Rocky and Rich in Water, New Research Uncovers, Space.com, 27 Jan. 2020, <https://www.space.com/41714-water-rich-exoplanets-trappist-1-system.h​tml>.​
Relevance	This article is relevant as it attempts to simplify the methods of discovering their exoplanets and gives a general explanation of what a TTV is, which is one of the main methods of determining features of exoplanets that are light-years away.
Authority	The author, Nola Taylor Redd is a contributing writer for space.com and has a Bachelor's degree in English and Astrophysics giving her authority to write such an article. She contributes to most of the astrophysics related articles published by the site and is credible as she is a recognised member of the space.com institution.
Accuracy	The accuracy of the article (Redd, N 2018) can be somewhat doubted as it includes spelling mistakes and some small grammatical errors that hint that the article may not have gone through a rigorous editing and checking phase. This might relate to further errors that have been found in this article. In this regard, the article (Redd, N 2018) states that all of the exoplanets lie in the stars habitable zone when in fact only three do. This might have been a typing error but in the end, it was missed and made it to the final published version, degrading both the accuracy and credibility of the report.
Reliability	The report is based on the findings of a scientific investigation (Grimm et al. 2018) that covers the "Nature of the TRAPPIST-1 exoplanets" which discusses atmospheres, masses, radii and the general composition of exoplanets. The article (Redd, N 2018) uses this information to compile a summary of the investigation into a more reader-friendly format.

Source #2	University of Liège, (2018), What have we learned about TRAPPIST-1 during this last year?, University of Liège, 27 Jan. 2020, <https://www.news.uliege.be/cms/c_9795836/en/what-have-we-learned-ab​ o ut-trappist-1-during-this-last-year>.​
Relevance	This article (University of Liège 2018) is somewhat relevant as it provides both an in-depth history and context of the TRAPPIST-1 system, but does not answer the research question. This is because it does not address the technologies used to find these exoplanets, only briefly referring to the telescopes used.
Authority	Throughout the article (University of Liège 2018) no author is mentioned, only the institution that published the article, which is the University of Liege. This makes the source unreliable as there is no way of finding out if the author is an expert in the field in which they are writing.
Accuracy	The accuracy of the report (University of Liège 2018) can be somewhat doubted as it includes a few typing errors, hinting that the article (University of Liège 2018) may not have gone through a rigorous editing and checking phase, meaning the article might not be as polished or as professional as it should be. There is also no clear bibliography, but many sources are referred to in the in-text links.
Reliability	This article (University of Liège 2018) is considered reliable as it is released by a university. This makes it appear professional and gives credibility. This article also makes strong references to a scientific investigation (Burdanov et al. 2019) carried out by researchers from the university who are led by the astronomer Michael Gillon. This makes the article reliable as it was based on the empirical analysis of a report (University of Liège 2018) performed by reputable researchers.

Source #3	Northon, K, (2017), NASA Telescope Reveals Record-Breaking Exoplanet Discovery, NASA, 27 Jan. 2020, <https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batc​ h -of-earth-size-habitable-zone-planets-around>.​
Relevance	This article (Northon, K 2017) is relevant as it makes multiple references to how the TRAPPIST-1 system has been investigated and gives detailed explanations of how the system was discovered and how other organisations intend to investigate the system further using newer technologies and equipment.
Authority	Karen Northon is the author of this article (Northon, K 2017) which was published by NASA, this sets a high standard for the article (Northon, K 2017) and provides readers to put a lot of trust in the material presented. Norton is a "Media Relations Specialist" with a Bachelor of Science, this gives the article authority as she has a background in the topic being discussed. NASA is a very reputable institution in the field of astronomy and gives this article credibility as the author has been vetted by NASA as an authority in astronomy.
Accuracy	In the fifth paragraph, Northon, K (2017) says that originally three exoplanets were discovered, and then two were confirmed with another five being discovered to make seven. This is very misleading and is a small error that can change the meaning of a large portion of this article (Northon, K 2017), especially when it is published by an organisation such as NASA. In the first paragraph Northon, K (2017) states that the first three exoplanets were discovered by the Spitzer space telescope but then in paragraph five states that researchers using TRAPPIST discovered the initial three exoplanets. Apart from these errors all the material included is linked and in clear terms making the information presented appear accurate.
Reliability	This article (Northon, K 2017) seems to be reliable as the organisation that has published this article (Northon, K 2017) is one of the leading institutions in space exploration. This article (Northon, K 2017) has used scientific investigations but does not reference them in a bibliography, so it is often unclear as to where the information has been sourced.

Source #4	About TRAPPIST-1, (2018), TRAPPIST-1, Trappist.one, 27 Jan. 2020, <​http://www.trappist.one/#about​>.
Relevance	This piece (About TRAPPIST-1 2018) discusses the techniques and equipment used to originally discover the exoplanets orbiting the TRAPPIST-1 host star. It summarises the TTV technique and explains why it is so much more effective when used on ultra-cool dwarf stars.The article also goes into detail about how astronomers might go about discovering new systems having exoplanets. This article (About TRAPPIST-1 2018) addresses the question and provides a detailed, yet concise explanation of how techniques and equipment are used to discover exoplanets.
Authority	There is no author mentioned for the article (About TRAPPIST-1 2018) and no publisher. However, it becomes apparent that the article is written and published by a team of researchers that originally discovered TRAPPIST-1. This gives the article (About TRAPPIST-1 2018) authority as the (supposed) authors are the astronomers who made the original discovery.
Accuracy	The information is very accurate and is free from errors, this suggests that the article (About TRAPPIST-1 2018) has been proofread extensively and is well written. There is also an extensive bibliography listed with nine references to scientific investigations and ten other references to various news and articles covering the topic. All of the statistical information is correct and is based on reliable sources.
Reliability	The article (About TRAPPIST-1 2018) is well constructed and well written, it is published by a well-known group of scientists which makes the article reliable.

Impact of the investigations

Throughout this article, many different sources have been analysed and assessed to attempt to answer the question, "How are potentially habitable exoplanets discovered and investigated through the use of technologies that utilise gravity and orbital theory, by investigating the Trappist-1 system". To address this question, sources should have discussed the discovery or investigation of an exoplanet using a technology that takes advantage of gravity or the orbital theory. All scientific investigations did so with an extensive and yet concise analysis of the TRAPPIST-1 system by utilising TTV (Grimm et al. 2018), spectroscopy (Wit et al. 2018) or photometric techniques (Burdanov et al. 2019).

Each technique aimed at discovering/investigating a different aspect of an exoplanet in the TRAPPIST-1 system. All of these techniques use gravity or orbital theory in some way. For example, TTV uses both theories as it measures the time a transit of an exoplanet takes to perform a full rotation around its host star. Spectroscopy uses gravity as it is measuring the atmosphere which is held down by gravity enabling researchers to calculate the amount of gravitational pull and the elements of the atmosphere. Photometric techniques rely on the fact that an exoplanet orbits its host star. As the technique is too light-sensitive it will therefore only be able to take measurements when the exoplanet orbits closest to us around its host star.

This alludes to the next point of discussion. Most, if not all, of these mentioned techniques can only be used on systems where the host star is either really small or the exoplanets orbiting it are really large. This is the case because the current technology is unable to measure such bright objects at such distances. This means that the "pictures" that are taken are often too blurry to distinguish if an exoplanet has orbited or not. This is an underlying flaw in all of the mentioned reports as they all use either the same or similar, techniques and equipment.

Size comparison of TRAPPIST-1 vs well known planetary bodies in our solar system. (Furtak, O 2017)

Future directions of research

Researchers are now awaiting the James Webb Space Telescope, which is set to launch sometime in 2021. This telescope will be able to confirm the exoplanets water content as well as reveal the compositions of the surface of exoplanets outside of our solar system. The telescope may even be able to detect signs of life on other exoplanets such as TRAPPIST-1. This telescope is planned to be positioned above the earth's atmosphere where atmospheric interference will not affect the Infrared spectrum allowing the telescope to peer deep into space and easily detect the elements that makeup exoplanets and stars like TRAPPIST-1. (University of Liège 2018)

James Webb Telescope Primary Mirror (Billings, L 2017)

Another future direction of research would be using the upcoming European ground observatories to monitor the system. This array of telescopes includes the ELT which is set to open around 2025 and would enable ultra high-quality pictures of the universe and be able to store that data in servers which would mean that cosmic events would be able to be replayed and analysed to a great extent. This will allow the further analysis of TRAPPIST-1 in higher quality and might even enable researchers to see the exoplanets orbiting the host star in detail, further adding to our knowledge of the TRAPPIST-1 system. (Wit et al. 2018)

Picture of SPECULOOS Southern Observatory located in Chile. (SPECULOOS Southern Observatory 2017)

SPECULOOS, which is currently under construction, will be able to observe more than ten-fold the number of star systems TRAPPIST could and at a greater precision as well. This telescope is the successor to TRAPPIST and aims to find dozens of star systems similar to TRAPPIST-1 in which there are multiple exoplanets orbiting in the habitable zone of their host star. This telescope will be able to sense near-infrared which will allow it to determine the atmospheres of exoplanets it finds. (Mathewson, S, 2018)

Bibliography

Scientific Investigations

(​Burdanov, Lederer, Gillon, Delrez, Ducrot, Wit, Jehin, Triaud, Lidman, Spitler, Demory, Queloz and Groote​)​, 2019, ​Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared​, ​(​Astrobiology Research Unit, NASA Johnson Space Center, Cavendish

Laboratory, Department of Earth, Space Sciences, School of Physics & Astronomy, The Research School of Astronomy and Astrophysics, Research Centre for Astronomy, Department of Physics & Astronomy and University of Bern​)​, 27 Jan. 2020, <​https://orbi.uliege.be/bitstream/2268/237500/1/1905.06035.pdf​>.

(​Grimm, Demory, Gillon, Dorn, Agol, Burdanov, Delrez, Sestovic, Triaud, Turbet, Bolmont, Caldas, Wit, Jehin, Leconte, Raymond, Grootel, Burgasser, Carey, Fabrycky, Heng, Hernandez, Ingalls, Lederer, Selsis, Queloz​)​, 2018, ​The nature of the TRAPPIST-1 exoplanets​, ​(​Department of Earth, Astrophysics Group, Space Telescope Science Institute, Space sciences, Laboratoire d'astrophysique de Bordeaux, University of Bern, Astrophysics Division of CEA de Saclay, Observatoire de lUniversite de Gen´ev, NASA Johnson Space Center, Jet Propulsion Laboratory, Division of Geological and Planetary Sciences and School of Physics & Astronomy​)​, 27 Jan. 2020, <​https://arxiv.org/pdf/1802.01377.pdf​>.

(​Wit, Wakeford, Lewis, Delrez, Gillon, Selsis, Leconte, Demory, Bolmont, Bourrier, Burgasser, Grimm, Jehin, Lederer, Owen, Stamenkovic and Triaud​)​, 2018, ​Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1, ​(​Department of Earth, Astrophysics Group, Space Telescope Science Institute, Space sciences, Laboratoire d'astrophysique de Bordeaux, University of Bern, Astrophysics Division of CEA de Saclay, Observatoire de lUniversite de Gen´ev, NASA Johnson Space Center, Jet Propulsion Laboratory, Division of Geological and Planetary Sciences and School of Physics & Astronomy​)​, 27 Jan. 2020, <​https://arxiv.org/pdf/1802.02250.pdf​>.

Websites

About TRAPPIST-1, (2018), ​TRAPPIST-1​, Trappist.one, 27 Jan. 2020, <​http://www.trappist.one/#about​>.

Amazing-space.stsci.edu, (2020), ​Figure: What does a spectroscope do?​, N/a, 28 Jan. 2020, .

Anderson, P, (2019), ​Clouds for some Earth-sized TRAPPIST-1 exoplanets | EarthSky.org​, Earthsky.org, 27 Jan. 2020, .

Billings, L, (2017), ​What Will NASA’s Biggest-Ever Space Telescope Study First?​, Scientific American, 27 Jan. 2020, <https://www.scientificamerican.com/article/what-will-nasa-rsquo-s-biggest-ever-space-telesco​pe-study-first/>​ .

Carne, N, (2018), Researchers reveal a little more of the TRAPPIST-1 story - Australia's​ Science Channel​, Australia's Science Channel, 28 Jan. 2020, .

Exoplanet Exploration: Planets Beyond our Solar System, (2020), Exoplanet discovery: Seven​ Earth-sized planets around a single star​, N/a, 28 Jan. 2020, .

Furtak, O, (2017), Comparison of the sizes of the TRAPPIST-1 planets with Solar System​ bodies​, www.eso.or​g, 29 Jan. 2020, .​

Haghighipour, N, (2015), Planet Detection: Transit Timing Variation​ ​, N/a, 28 Jan. 2020, .

Kelley, P, (2018), Study brings new climate models of small star TRAPPIST 1's seven intriguing​ worlds​, Phys.org, 29 Jan. 2020, .

Landau, E, (2018), 10​ Things: All About TRAPPIST-1​, Exoplanet Exploration: Planets Beyond our Solar System, 27 Jan. 2020, .

LinkedIn, (2020), Karen Northon​ ​, LinkedIn, 28 Jan. 2020, .

Mathewson, S, (2018), TRAPPIST-1 Planet May Be Wet and Life-Friendly​ ​, Space.com, 27 Jan. 2020, .

NASA, (2017), Seven planets orbiting the ultracool dwarf star TRAPPIST-1, ESO, 29 Jan. 2020, <https://www.eso.org/public/unitedkingdom/images/eso1706p​ />​ .

Nave, R, (2020), Kepler's Laws​ ​, Hyperphysics.phy-astr.gsu.edu, 27 Jan. 2020, .

Northon, K, (2017), NASA Telescope Reveals Record-Breaking Exoplanet Discovery​ ​, NASA, 27 Jan. 2020, .

Phys.org, (2018), ​Research reveals more about TRAPPIST-1 planets, and the possibility of life, N/a, 27 Jan. 2020, .

Redd, N, (2018), ​TRAPPIST-1 Worlds Are Rocky and Rich in Water, New Research Uncovers​, Space.com, 27 Jan. 2020, .

SPECULOOS Southern Observatory, (2017), ​SPECULOOS Southern Observatory​, ESO, 27 Jan. 2020, .

ScienceDaily, (2018), ​Atmospheres of exoplanets in TRAPPIST-1 habitable zone probed​, ScienceDaily, 26 Jan. 2020, .

University of Liège, (2018), ​What have we learned about TRAPPIST-1 during this last year?​, University of Liège, 27 Jan. 2020, <​https://www.news.uliege.be/cms/c_9795836/en/what-have-we-learned-about-trappist-1-duringthis-last-year​>.

Urrutia, D, (2020), ​An Alien Solar System: TRAPPIST-1 Discovery Tops Our 2017 Exoplanet List​, Space.com, 27 Jan. 2020, <​https://www.space.com/39211-trappist-1-exoplanets-solar-system-discovery-2017.html​>.